i had £900 worth of exhaust; £300 of intake; £400 of High Flow Cats. thats £1600 roughly and i doubt i made more than about 6bhp; but then i had not been remapped, but again i reckon i could double that figure to about 2 bhp but add on nearly another £400
so nearly 2k for not a hugh difference in performance. the noise was good, your half way to a supercharger at that price.
if your going to spend then another say 5-7K on a supercharger just have a think about what car you could buy with another 7k which would have the performance.
i'm not trying to pi$$ on your chips here, but think seriously before you start modding, if your doing it for the noise then knock yourself out, but if your only doing it to chase BHP, then my advice is
get it remapped as it is, maybe with something for the noise, and then go do some driver training and track days. you'll make more gains from the performance already there than by adding 2k in mods.