What's your budget?!?! Prices quoted below are very approximate and sometimes on the high-side (I have expensive taste and am v picky with garages).
Induction (?60, but ?100-?160 fitted), combined with a decent stainless cat-back exhaust (my Milltek beauty was ?275+VAT, fitted) is a good start.
Gas flowed head (approx ?900+VAT, fitted) / high lift cams (around ?700, fitted) are great, but pretty expensive, even if you go DIY / get somebody from the site to help you, they're not cheap! I think I was looking at ?400 + labour to do the cams myself, but ?700 got a good garage, a guarantee and remap.
Once you've done as much / little of the above as you want, then get it remapped to suit your mods. Best value for money, but get it done afterwards. Better fuel economy, much smoother power... plus a few bhp extra
Cost anything from ?30-?350, recommend spending ?100-?150 and getting a dyno (rolling road) run at the same time.
Mine was 142bhp & 148lb/ft with the 1st remap, PiperX induction and Milltek Jet cat-back exhaust... not sure now (cams and new custom remap), but guessing 155+ @ the flywheel, hoping for 160ish when I give it a power-run in a few weeks!
Standard 8v is 115bhp & 122lb/ft, so not soing too badly.
