With the C20LET its possible to go to 260BHP with a simple plug in chip (not superchip) called a Phase I
After that, the Phase II (280BHP) and Phase III (315BHP) require the addition of a front mounted intercooler which comes supplied. The only problem you may have here is if you have the standard SRi bumper which is tapered back the way menaing the intercooler would be right out in front.
Swap for a GSi/Turbo front bumper (or other aftermarket one) and you'll be sorted.
If you wanna go daft then the Phase IV (350-380BHP) is available including uprated short block and larger compressor but with 2WD you'll just end up spinning lmao