1g's can make power but due to displacement just not as easy
i would start with upgrading the downpipe... i think 3" is a little too big for a 2L
maybe 2.5" from turbos back
oh and of course before exhaust.... have you got an air filter on there yet? pod filter...
cold air = more power... so you could build a cold air box and get some ducting pipe from the front bumper to the intake
a huge restriction i believe is the intake piping to the turbos... if those were remade i'm sure there would be a big flow difference
careful of overboosting those turbos also.. they're only ceramic and like dying hehe
since you have a 1990 i would assume this is a generation 3 ecu which is either a 9psi or 11psi model... i would work on things around the engine first
so intake, exhaust.... cold air...
upgrade the intercooler & intercooler pipes (custom jobby tho... not much aftermarket support for the 1g)
that's bout all i can think of right now
p.s. ur lucky you got a 1990 1g.. if you had gotten an 89 it would have been narrowbody lol