I'm with stupid ^^^
I wouldn't go higher than 12psi on a stock turbo ... and because stags run rich from factory, that'd be the limit before you need to get it tuned. But heck, if it was my car, I'd be getting it tuned, or at least the AFRs checked at 12psi. Also, have the timing checked too.
I suspect your biggest problem with the car is the stock exhaust not letting the engine breathe better ... and the FMIC adding lag to the stock turbo. Get a 3" system, HKS 2530 turbo, 12psi boost, SAFC Neo, a tune and you'll be much happier with it.
I recently did a manual conversion to my Stagea. I had all the bits ready ... read everything I needed to ... and there will always be something you'll miss. I had my stag off the road for 2 weeks, taking my time, not rushing and doing it right. And I'm stoked with the results ... the stagea is awesome to drive now. Highly recommended ... cost me about $1900 to do (that included a Nismo clutch kit and lightened flywheel). I have no faith in the tiptronic auto rubbish anymore.
BTW where are you located?