Just out of interest how quickly does it drop back to 12psi after its spiked? If its more or less instant it could be that big black reservoir on top of the rocker cover pissing it all out due to a boost deviation? I *think* thats the n249 doing what its meant to
Id be checking all vac lines and getting rid of all the unneccessary crap under there first bud
(n249 bypass at least and possibly the n112 aswell but youll need some resistors to do the full job)
My car spikes to ~20-22 psi which it holds for just over 1000rpm then tails right off to 10psi by 6k and slightly under that at 7k. Mines meant to run 200-210bhp (previous owners quotes) but im pretty certain its well under that. Maybe 190-195 at a push, im happy though as its running really well for the first time since i got it!
Just the limitations of a standard exhaust, smic and TIP