Race gas is never determined by boost alone. Right now I'm running about 17-19 psi on pump on a T66 strapped to a 1JZ with A/Fs in the early to mid 11s. Zero knock and it's never done anything to scare me while driving it.
Here's how you should relate the need for race gas. 1. Your horsepower vs displacement/engine layout. 2. Compression/cylinder pressures. 3. What you are using to tune with/who is tuning. 4(optional, haha). If you are at the ragged edge of your injectors and want just a little bit more temporarily.
A good rule of thumb with race gas is any time you make over 80 hp per cylinder, it's a good idea to consider over 100 octane unless your tune/tuner is very good. Mind you again, this is all displacement dependent. Smaller pistons/displacement will fight knock better than a larger piston.
I can't imagine that you would make any more than 450-500 wheel with the set up above at the boost levels you are requesting, however as stated above what size injectors and what are you using to tune? I made about 400 wheel with a 60 trim on pump gas at about 23 psi with a 4g63. After that, the car got tuned on race gas as that's pretty ragged with pump on 4 cylinders(with old tech; s-afc and MAFT). Hopefully this helps a little bit.