Porsche brakes for £700 is doable maybe a bit less.
They are sizeable calipers and can be made to fit effectively, here are some GT3 RS calipers on a Mk4, these are over 90mm wide but with some wisely chosen wheels and some adaptors we managed to get them on a mk4 with no poke
Ive been toying with the idea of doing a carrier for 312's on boxster fronts calipers which would be a cost effective 1/2 way house between the S3/R32 stuff and a Fullon BBK with a little Bling thrown in cos we like it. Theres good pad choice for that caliper and the boxster fronts have the same piston surface area as the 996 Carrera 2/4 calipers but are limited to a 25mm thick disc so it would work well.
If anybody wants to guinea pig it for me I'd do it as cheap as I could, but it wouldn't really validate it unless somebody was tracking it with some 1/2 decent pads which aren't cheap. The geometric limit for the caliper is about 320mm on disc diameter and we all know we like to think biggest is best which again we all know isn't really the case. The 996 only gets 318mm brakes and they're phenomenal.
The big problem with the S3/R32 kit is pad availability - theres just not much out there for them, Beddie brought my attention to some Carbon Metallics for a Mazda Race Series that would fit but they contaminate your disc when it comes to your driving to work pads
PM me if you fancy guinea pigging that setup.