Assuming the 500 watt amps really put out 500 watts, then yes, they would be sufficient. I use 650 per Kappa Perfect and it's sufficient, and 500 is barely less than 650, from a log10 function basis. However, if you do plan to build a 2nd matching sub later, I would recommend getting an outboard pro amp as opposed to plate amps.
There are very limited measurements of the MFW-15 and A5-350 available. But based on those that are available, the Kappa Perfect used in a cabinet built exactly to my specification would very likely produce substantially higher output in the lowest range(20Hz range), on the order of 4-6dB higher output. In the higher bass range(30Hz on up), the SPL would be very similar. As for distortion: the Kappa Perfect VQ uses an extremely linear motor that would keep distortion lower into higher SPLs, as long as the Kappa Perfect is not operated beyond it's linear excursion range(exceeding roughly +/- 14mm will enter it's non-linear range). Due to the Perfect's linearity, it will sound essentially identical regardless if used at 85dB or 105 dB, unlike many subwoofer that start to distort substantially and exhibit compression artifacts when they are used at higher SPL ranges vs. low SPL. Again, you should refer to avaserfi, as he is a 3rd party and has measured the unit at high SPL ranges and used it in high SPL use(110+dB range). Just ask him in this thread, or via PM, if you want opinions by someone that built the cabinet to my specifications. The additional beauty of the Kappa Perfect VQ besides it's high linearity, is that it has the variable motor that can be adjusted to work in any type of bass alignment. As such, the driver has a lot of future applications if you decide to replace your sub with something else later. In fact, I would recommend getting two of the Kappa Perfect drivers, so that you can build a second matching cabinet in the future.
-Chris