I was like "B7W" what is that? Then I got it...
OK...
Ceiling speakers aren't optimal, and you have high ceilings so I would stare clear... Take it from me, I have a dozen ceiling speakers and they are good for ambiance, but thats about it...
Artisians are a good option if they will work for you, room wise, it looks like the tv is in a small area in your pic...
I'm confused about your budget, is it $300 per speaker for all 5 so that is $1500... or do you want to keep your center and buy 4 speakers? so lets address that first, If you keep that center, you need to get a matching front and right speaker set, from Polk, Im not positive what would match but Im thinking it wont be what you want aesthetically...
How about this... Not sure if this is in or out of your budget...
You can piece together a sound bar system... this is what I would do if it were my money...
these were over $1000 not that long ago...
BOSTON ACOUSTICS P400 On-Wall 3-Channel Speaker Black | Accessories4less
these wont be too noticible on the white ceiling
BOSTON ACOUSTICS MCS130 Horizon Series 2-Way Surround Speaker Each | Accessories4less
And then this sub is just begging to be bought for the money its really nice, on clearence-
Newegg.com - Klipsch Reference RW-12d 12" Powered Subwoofer Each w/ New box
That would only be $850 or so, and you get a 5.1 {sound bar has 3 inputs} with a good sub, really that systems MSRP is over $2600!!!
Also not sure if you have an avr already or what, but if it has too look small I like these marantz slimlines, If I was getting a sound bar, I would pick one of them, they are at a4l cheap heres one-
MARANTZ NR1402 Slimline 5.1ch AV Receiver w/HDMI & 3D-ready | Accessories4less
there are a lot of sound bars out there...
KEF KHT8005 Soundbar Speaker System | Accessories4less
Good price on that unit, and with the right ceiling mounts them little speakers will seam invisible
kind of...
I have heard the 8003 sound bar and it is nice...
That is by far not the best system for the money, since for that kind of money you can get a set of decent speakers with a decent sub, but you obviously realize you are going to have to make compromises for the appearances you want... Im not sure which sub you have, but most likely it isnt more than a 200w 10" Polk, the kef would probably be on par with that, and you could then sell your entire Polk system as a whole...
One last thing, I would mount the surrounds from them ceiling boxes with a light transition kit and long rod, match the light ceiling hub and rod to the one on your fan and then hang the speakers that way... But the wife probably wouldn't appreciate the ingenuity...