again id have to disagree, weighing in at little over 750kg fully loaded and with 80 or 90 bhp easily achievable (sp) you will be getting somewhere in the region of 106-120bhp per ton and thats without any stripping out even if you got it down to 700kg theres a potential 128.5bhp per ton.
just to put it into perspective,
a stock mk2 golf gti is according to the beb around 1100kg upto 1245kg depending on trim level and extras. they have a 1.8 engine producing around 110bhp so there power to weight is 100bhp per tonne, i know its not a 'fast car' but its of similar size (roughly) with a far bigger engine,
my k10 with 1.2 engine, 4-2-1 manifold, bike carbs could clean the floor with my mates mk2 golf upto 65-70mph, obviously after that, his increased torque pulled past me but at that point your speeding so isnt a fair test really.
depends what you want, if you want to sit at 200mph save up for a bugati vayron,
if you want a quite fast nippy little car, then stick with the micra, my 4-2-1 manifolds are about £100, the bike carbs i had were a similar ammount, so for £200 there are quite some good increases to be had.
perhaps if money or age wasnt a factor i would have gone with a bigger engine from the start and done the same mods to it, i was looking at a ga16 1.6, stock 90bhp, so more that ny tuned ma12, plus id be set to gain 20-30 bhp from a few simple mods, but hwen i was 19 and looked into insurance on a k10 with a ga16+mods i couldnt even get a quote never mind any sort of cover.