Audi S3 has a k04 turbo (k04-001 i belive) which is a streight swap i belive.
i also belive the mitsubishi evo (cant remember which year) has a turbo thats a relative streight swap and belive one of the scoobys uses the same turbo and costs alot less.
the k04-001 is proberbly the easiest turbo to fit.
in essence you can fit any bigger turbo you want, its just youll have to look into things like custom manifolds/ different manifolds, maf sensors, throttle boddies etc.
i belive toby put a k04-025 on his golf, but he spent a fortune on his, think he fitted a relentless manifold, badger 5 throttle boddies different mass air flow sensor bigger cooler etc etc etc.
if your really that interested in fitting a bigger turbo then drop him a pm, equally send nick or someone from rtech/carbon chip a pm on here, sure they will talk you through some stuff. equally you could email a local turbo specialist, they will know what fits and what doesent and any work required.
but as stated, fitting new turbos comes after you have done other stuff imo.
start off with full exhaust system(not just cat back), cooler, boost pipes, diverter valve, injectors, custom maps. look at induction (either induction kit or pannel filter, have to weigh up the pros and cons and see whcih you think is best), bigger indiction (3" maf etc.) then look at extra cooling WMI etc. then before you start to look at bigger turbos look at what you want to replace first, what power and boost you looking at, will it need new rods, will it need new cams etc etc etc.