GA15 is the vertically mounted throttle body right? (i.e. facing up), i know the GA16 is regular horizontally mounted, try getting a GA16 intake manifold, make an adapter plate ( not sure if its necessary) and use the GA16 intake manifold, it should have a slightly bigger throttle body, and be mounted for easy intercooler pipe fabrication as well...u should also look into getting..
-metal head gasket
-rising rate fuel pressure regulator
-fuel pump with bigger flow capacity
-if its a GA15, try getting injectors (or an entire fuel rail) that will work, that has larger injectors, not sure if GA16 will work..
-make sure ur ignition system is up to mark, get some NGK iridiums if u can atleast...i would retard timing a bit as welll just to be safe..
-get an air/fuel gauge, so atleast if u see the engine starts to run lean u can back off and save it or reduce damage..
-make sure your vacuum lines on the wastegate etc are good and not slack, u dont want any boost spike @ 10/12psi on a GA with stock internals...not nice..
-if u cant get at least a piggyback to 'tune' ur car, i wouldnt recommend going past 10psi..
my 2cents, that said, i hope u not going street racing eh pal