I live in the US and I can tell you for sure the z22se (known here as the L61) is quite capible
I have a Veccy B disguised as a Saturn and it is making respectible power about 140 at the wheels thats 160+bhp at the crank. you are fortunate in that you live in the UK and tuning is readilly availible for your computer. I would recomend a 4-2-1 manifold followed by a 2.25 inch exhaust with sport cat a resonator and a nice muffler. Now the intake side there are so many routes you can go but easiest is a large bore throttle body and a free flowing inlet pipe with a good filter(K&N, pipercross, viper, etc...) you would be looking a a feelable increase of power and a nice sound. NOW get it tuned on a rolling road (regal or cortnaysport) you will see a similar jump in power. I have no doubt you can see 175+bhp without opening the engine.
if you want more there are: cams, IRTB's, superchargers, turbos, and full on engine swaps.
it all depends on how much you are willing to spend