My previous ZS 180 had the following modifications. I've put costs next to them too, but prices have altered since 18 months ago.
These are just the prices I paid - people like George may be able to do cheaper.
Janspeed manifolds - £575 plus £250 fitting
Janspeed sports cat - £250 plus £35 fitting
Janspeed back box - £175 plus £20 fitting
Janspeed intermediate pipe - £140 plus £35 fitting
(I did my exhaust over a few months so had to pay fitting for each individual item)
ITG Maxogen - £364
Dastek unichip piggyback ecu - £464
Piper cams and pulleys - £250 forum private sale
Fitment of cams, pulleys and new belts - £900
After all that the car was finally rolling road tested at 204bhp before I sold it.
I'm going a slightly different route with my new ZS. There's a decat on it's way, and this time round I'm going for good brakes, lightweight wheels and lightened flywheel before looking into reprofiled cams in the autumn.
In the grand scheme of things it's very small gains for a lot of money.