For a fairly small improvement you could look at the usual performance air filter and exhaust combination (which isn't changing the engine itself).
You could look at lightening the car E.g. Stripping some of the interior like the rear seats, carbon bonnet (£Â£Ã‚£'s), remove the spare wheel etc. etc.
A rolling road tune-up would maximise what you've got.
Better suspension, tyres and brakes won't give you better acceleration, but you can go faster for longer!
