Standard back box is good for up to 400bhp by all accounts, it is the Y pipe that strangles the life out of your engine. Japspeed is 'okay' - the Stillen from Tarmac is widely accepted as being super duper though
Just tailor in your expectations on gains, I think a few 276bhp cars have gone on to get 300bhp with the above mods (plus full sports exhaust) but overall it will drive miles better.
I am in the process of doing a few mods, added a plenum spacer yesterday, put on a milltek cat back last weekend, berks and uprev on thursday!