Mark,
I reckon you just go for some bigger injectors and wind the boost up a little more - That will get where you want to be no problem at all.
Forget the Plenum, for the moment - it's a lot of money for only a bit of gain.
I share Blitz's thoughts too - be very very careful about who tunes it - they should have an outstanding reputation for tuning skylines.
As I have been reading and seem for myself first hand - When you go fiddling with too much more power, things can go phuuupppt.
Cheers,