When you say "a lot of guys get away with..." I wonder how accurate that is or perhaps more like "how long did they get away with it?"
The implications of making 500 WHP is boost at 26 to 27 PSI and at the upper power range the fuel lines can't keep up.
I did 373 WHP at 21.6 PSI and 93 octane. That was all my tuner was willing to do for a stock block track car. It popped the ringlands soon after when oil got into the intake. Basically, I think most builders and tuners agree that stock motor heads will not stay down if you go over low twenties of boost.
If you look at EFI Logic's (where I've always gone and who has a great reputation) comprehensive dyno database and enter "Subaru E85", and sort by WHP - high to low, you will see that for tunes above 400 WHP most engines are built motors and above 450 virtually all are to some degree.
On my first replacement motor I did the usual forged pistons and ARP studs on a new, factory shortblock. That engine was tuned by the late Chris Schoen-Kiewert for 24 PSI (on 100 octane) yet saw an occasional 26+ PSI when the wastegate of the Blouch 2.5XTR turbo was sticking. Ran awesomely then, BTW, but I'm convinced it wouldn't have for long. That engine had 70+ track only hours on it before I burnt two valves; probably because the Walbro 255's capacity started to wane after five years of use.
So, I believe that, without the proper mods, pushing a stock motor above 20 PSI is a recipe for disaster, regardless of fuel.