I believe 17psi is the max before you hit boost cut. even at that, i would be careful. I would wait for full engine management if i were you. as for HP increase on those 2 extra lbs of boost, i dont think it would be much.
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ya generally a turboed car would gain ~5-10hp per lb/bst , but this all is based on a LOT of different factors. Engine size, turbo size, etc etc. 2 psi would probably give you ~10 hp maybe, but I wouldnt recommend it without UTEC or an engine management of some kind (AFC / HKS AFR ...). Just appreciate the boost controller for allowing it to spool up 500-800rpms faster and leave it at that, that part of it is well worth it anyways.
well against better judgement, I know someone who has been running up to 19.5psi and only hits fuel cut occassionally. The car has TONS of power at that pressure and a full 3" free flow exhaust and single free flow cat.