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.
not the best advice then telling him to get a custom map before fitting a new turbo 
why not? if he gets WMI then the cooling from that is going to allow him to run more timing and boost, a custom map wil account for that were as a flashed map will not.
same with any other mods, the map could then be coded to suit how the car is set up. and can be coded to how he wants and include other stuff, launch control, left foot breaking (not sure if you can get it on mkivs but you get the picture).
obviously he will proberbly need another map once the turbos changed, but if hes going to go down an expensive route of manifolds, rods etc etc etc to fit a bigger turbo and go for big power it could be an on going project and the custom map will give him most power for the current set up, or he could do them mods and think actually its pretty fun as it is so I wownt spend all that money on a bigger turbo and go splash it on a few beers or a holiday or something...
