i think <500w asic miner market is underserved, and will be critical for increasing distribution of hash power. i will be eagerly awaiting what you are releasing next.
philipma1957 could do 810w with an S9 running at 10.5TH, so.... take out one/two boards for 540/270 3.5/7TH...
As for ATX PSUs, you are supposed to use no more than 80% (and only the best ones are efficient at that load, with most peaking around 70%) so if the gear actually needs 500w think about getting a (decent brand) 750w PSU...
Which often ends up hilariously more expensive than getting an asic miner psu and sub use it (ie. Bitmain's APW, etc).