Is it just numbers you're chasing or actual performance improvements?
If it's performance to make the Zed/you go faster then you could look at better tyres (depending on what you've got already), suspension improvements, improved brakes, some driver training etc.
For actual Bhp improvements I think you've pretty much covered it before you start having to spend a fair bit of money for not a lot of an increase. For example you could get a nice set of exhaust manifolds (£400 & upwards) for a few more ponies but fitting will cost you a lot at a garage ~ usually easiest to have them fitted with the engine out of the car.
Something else you might not have considered as a good performance increasing upgrade is a lightweight flywheel and new clutch kit. A lot of people state a much improved drive afterwards as well as an improvement in acceleration. More noticeable I'm sure if you've an old worn OEM clutch & flywheel unit.
Whatever power mod parts you add though (apart from clutch, suspension etc) you'll need your UpRev map tweaked to account for the new mod (like manifolds) so that's a bit more money on top again.