If you are thinking of bigger turbo, keep in mind the whole package.
Still I wouldn't be too scared of taking big enough. Even if everybody is saying about lack of torque in loewr gears, with right kind of set up there is not too much of worry about.
I have K27/32 turbo, but also lots of RUF Racing Stuff in it:
-C-group cylinders and pistons
-RSR-12plug ignition system
Of course there is 3 oil coolers in front, 3 gas-pumps etc.
pushing nice 580rwhpp and +600rwNm
Actually in this street legal car there is not too many things which are not modified.
FUlly Adjustable Fox Racing Shox suspension with 935 everything under, 350mm Big Reds, 1040kg kerb weight, full FIA-GT spec.
Full Strosek body (with self made front spoiler and side skirts), lexan windows.
The whole package is now in excellent balance.