17/18 psi on a 28RS with cams and a extrude stock manifold should get you in the 340-360 whp range...
At that powre level, your 550cc injectors are just about maxed out...you still have some room, but I'd say 350 whp about the limit of them.
Lowport SR, gt28rs, GTiR manifold, 40mm external gate, 50lb injectors, Cobra MAF, JWT tune, JWT S3 cams