Stainless steel free flowing exhaust with sports CAT and manifold? Without the
manifold it will be (should be) cheaper than a Brab or Remus backbox - they don't add
power just noise and weight.
ITG, K&N, Pipercross et al will add a modest increase in power, a couple of bhp.
Maybe an induction kit with NACA duct on the bonnet for a cold air feed.
As for fuel choice, V-Power won't add much to a forfour petrol engine, BP Ultimate
does the same job (imo) - cleans the engine and gives an all round better response.
I believe you can remap the N/A engines, I think BIG does it but only adds a few bhp
maybe a max of 15bhp. (or should that be 1.5bhp !)
Apart from that, you could strip the interior, the aircon, electric windows, carpets,
soundproofing etc etc and roll on magnesium alloys.
Thats my thoughts...