My friend have a standard turbo R32GTR that ran 11.9 on street tyres with just PFC, twin plates and usual mods like zorst, filters 16psi.
I personally have same car that ran 12,4 again on street tyres with same mods only 2psi less (14). I belive my car was slower due to my clutch was sliping, my tyres size are uneven meaning by the end of the terminal i still have about 30% power to the front where i'm not suppose to be.
You don't REALLY need bigger turbos to beat Sti's or XR8, well having said that doesn't mean you'll beat everyone of them, there is always someone out there that faster than ur car never the less.
If you really want to upgrade then i'll say go 2530's or R34N1 from what i've read on the forum and from what i've been advise by most of us VIC Skyline's owner's tuner which is BEN from Racepace.