If yer wanting to up the performance yer gonna have to drop something
In my 6mps Im averaging 21l/100m I think is just under 22mpg, yes its been higher on runs but Im not one for trying to delude myself that I get awesome MPG in what may be considered a hot hatch/saloon.
The 3's do far far better, I guess it's because of the extra weight and the loss thru the AWD on the 6, think yer looking mid 30/high 30s
The 3's are a rather enjoyable and tbh seem to be dropping in price, 6k would see you in a well sorted Mk1, possibly a stock Mk2, which are pretty nice inside imho, far prefer the seats to the one in my 6 too.
Theyre hungry on fuel and a one plated after early 06 and yer looking at 510ish a year on tax alone

Or the Mk2's are int he same lower band as the earyl 06 ones, 260 a year I think.
The =civics linked above my mate has a 2.2 CtDI Type S, the stock suspension is bone shatteringly harsh

He's just had the springs changed for Eibachs and whilst a bit better its still a bit harsh imho
Those Octy VRS's above look nice and I'd happily have a try in one tbh.
I've always like the old shape astra's why not go something at the cheaper end and mod it a bit?
As above, a remap on most diseasels sees good returns too, had a think about that?