Just buy a fast car in the first place..... I bought a car for £11,400 with a full warranty, reasonable insurance etc etc and it was fast as standard. Why buy a slow car and try and make it as fast as something that will have cost a little bit more, but would be standard and probably have cheaper insurance (not modded). Loadsa people say that says that the saxo handles better than the 172, but if you put a turbo or supercharger into it, youl be dealing with loadsa understeer etc.
As Jeremy Clarkson once put it, if youre going to modify a car, start with a fast one in the first place......
Only my thoughts/opinions tho.
A friend of mine bought the same year (Y-reg) as mine, but a saxo vts, which incidently he paid more for. Hes been spending money on it to make it go faster and is infuriated that he still cant beat me, even when im 4-up - a classic case of should have bought something faster. I suppose its the free insurance etc.