figure this, I've got 38s on my supercharged 3.8L, and that's with about 13LBS of boost from the factory!
don't go too big because when they cycle in a non-boost environment they'll spew rather than spray at lower fuel rail pressures.
what duty cycle are you planning on having the injectors spray at when you're at full boost? hopefully no more than 80%!
I'd go with the same with your turbo conversion for 300 hp at about 45 lbs rail pressure.
bigger injectors for turbos than S/Cs btw.