There's many, many factors involved in these things, not just the size of the driver.
For starters.... we have to actually know our system impedance and power, not just what the drivers are rated at.
1) How have you wired these drivers? Onto different amp channels? Onto the same amp channel in parallel? or onto the same amp channel in series?
All 3 of the above make a difference in the type of power the drive are seeing.
For example if the amp is rated at 50watts RMS x 2 into 4 ohms, there's three possibilities
1) You've got the 4 ohm drivers running one off each channel in dual mono - they're each seeing 50 watts RMS
2) You've got the amp bridged, let's assume we're getting 200 watts into 4 ohms bridged, but if you've got the drivers wired in parallel, your amp would be seeing a 2 ohm load, and trying to put out a TON of current, creating 400 watts; 200 to each driver. Chances are though, this would fry your amp, although in car audio a lot of amps are made to be stable down to 1 ohm loads and therefore can take a 2 ohm bridged load. You'd get a lot of power out of this but I would never try it IMO. If your drivers are 8 ohm though, this is safe to do as your amp would see a 4 ohm load.
3) You've got the amp bridged, and the drivers are wired together in series. The amp is seeing an 8 ohm load, and is therefore putting out enough current for 100 watts of power. This is recommended, but it'll of course give you the exact same output as running the drivers off of each channel.
So let's assume that with 50 watts of power per speaker, we've got a total of 100 watts of power being put into the drivers and being transduced into acoustic energy. There's a few things at this point that will affect so-called "output" - it's not just how much power and the size of the driver... there's sensitivity for starters. Different speakers may very well produce different sound pressure levels given the same wattage and driver size. It's all about design tradeoffs - what do you want? Max power handling / Loudness? Sound quality? Efficiency? These are some of the things to look at when comparing two drivers.
That's just the driver though. A driver and a speaker are two different things. A speaker is a system - in the case of a one driver speaker like a sub, it's also the result of how the driver interacts with an enclosure. A vented enclosure can often sound very different from a small sealed enclosure, which in turn can sound very different from a large sealed enclosure - all with the same driver(s). You have to know what the right enclosure for your driver is (a good way, although not the end-all-be-all, is to divide your driver's Fs by its Qes and see if you're below 50 (sealed), 50-100 (sealed & vented) or above 100 (vented))
Now let's look at your drivers, the Rockford Fosgate P1S48.
these are an 8" driver with some of the following T/S parameters:
Fs - 40 hz
Re - 3.20
Vas - 13.9
Sd - 200 cm^2
Qes - 0.56
Qms - 6.90
Qts - .52
Sensitivity - 84 dB
I modelled it in a 2.5 cu ft box with a port tuned to 30hz - although i don't know how you've got yours. but with that box it can in fact take a full 150 watts of power as long as there's a 25hz high pass filter being used. a 30hz tuning looks very nice actually; there's a shallow rollow to the tuning frequency. Impedance does dip rather low at 3.3 hz at some points but i think car amps are made for this IIRC. Group delay does get kind of iffy but that's a given with such a high tuning and HPF frequency. To be honest though, I'm STRONGLY doubting your ports are tuned to 30hz. If I know car audio, your port is tuned to something silly high like 40hz or something like that... so with that I have to ask how long and wide your vent is. I recommend tuning it as low as you can, especially with cabin gain. If I had to give you a rough number based on the model,
-2.1 cu ft box with good bracing and well dampened
4 in x 19 inch vent for a 25hz tuning - you're giving up some SPL in the 30-40hz range but it's ideal in a car environment
-No more than 200 watts of system power (100 watts per driver)
Also before I bust out some 15" driver i'll just point out what that Sd value above means. An sd of 200 cm ^ 2 is the surface area of the driver cone. This is how much effective surface area it can move air with. An average 15" driver has around 900 as its sd. So a pair of your 8" drivers won't even come close to a 15 in terms of air displaced for the same distance of cone excursion - there's just less surface area. Your 8" would need around twice the excursion (in/out movement) to match a 15 and in order to get more excursion you need more power - it's all about efficiency. It would take around four of your 8 inch drivers to match that - but you have to ask what that means. Does that mean you would be getting louder, or just that you'd be getting the same loudness with more ease.
So with that said, let me just say that a 15 is way overkill for a car. It's overkill for a small room. It's big for a medium sized room. A 15 inch driver is huge. I'm of the opinion that even a 12 is overkill for a car. A 10 is just right - and even then this is in a sealed enclosure - cabin gain for a vented enclosure would just sound bad in a car.
If you want a new big subwoofer to get louder, yes a 15 in a huge enclosure will likely give you more raw output and higher power handling than a pair of 8s... a pair of 8s is close to a single 12 inch driver. Of course, adding more power to your 8s may also make them louder. again, i have to ask if loud is what you want. Also realize that larger drivers tend to want larger enclosures... a 15 may sound much too punchy in a small box.
But if you want your bass to sound more realistic, look into finding a nice 10" driver suited for a small sealed sub, deaden the crap out of your car, and go with that, crossed over to a pair of front speakers at no higher than 100hz, (if your mains can dig deep, I'd say even 60hz with a 24db/octave slope) preferably lower (80hz is ideal) and get fronts that can keep up with it. And if you want music to sound good period, adding more subs than what you already have isn't where I would start - first I'd deaden it to elminate rattling and outside noise, then I would add other types of interior dampening, and then I would look to scrap any 6x9 or coaxial or otherwise speakers in your van, and consider actively crossed fronts - a nice tweeter like the Seas Prestige 27TAFNC/G paired with the Exodus anarchy (LR4 both ways crossover around 1.8khz) might be a nice place to start - that anarchy will give you a ton of bass.
I would add a processor which can help clean up phase, gain, and reflection problems so inherent in cars, like the Alpine PXA-H100 - this thing has Audessey in its software. But maybe that's just me. I really do think the amount of bass you've got in your car is more than anyone should need. There's home theater subs out there that use dual 8 inch drivers such as the aperion 8D and these are meant to full an entire room with bass, not just a tiny little van.
Ps this is a home theater type forum so sound quality to me is more valued than "bumpin the block" than it may be on car audio forums. maybe you should look there where they'll tell you need four 18s in an infinite baffle if you just want the go-ahead to get bigger subs.
