True enough, and that's assuming the 200, 300 and 500 are measured the same way. What one manufacturer calls 300 another might call 500, and out of two amps rated 500rms at a given distortion level, one might be able to sustain that for 100mS and the other for hours.
Not sure where I'm going with this apart from "don't believe the hype", "get an SVT" and "1000 genuine watts are only twice as loud as 100 genuine watts"