As others have said;
Cams (either 438 or BTM's GT ones) induction, exhaust. I went for the 438's from K-Tec and I rate them a lot. I am a little concerned why you don't want the mods to be overly 'visible' as if it is to not declare them on insurance, then it is the last piece of advice i'll be giving you. I have zero respect for anyone who helps push my already significant insurance cost up through being a completely selfish tool.
Oh, and to back others, serious power gains = serious money. I went into the modding of mine in full acknowledgement of the cost involved. I'd recommend anyone else does the same. The bills don't hurt as much then, as you re-assure yourself you expected them
A full service (both belts), 438 cams at the same time, Full set of Grp N engine mounts and one or two other bits, plus labour, and I got little change from £2k. On a £2.5k car, that can be hard for some to swallow. I swallowed it, as I am building a track car that will hopefully provide some reliability (hence cams and ITB's, not charged) whilst being reasonably fun and cheap for more regular 'disposable' parts i.e. tyres, brake pads etc.