I have everything lined up for a V6 2.7TT swap (apart from time), really just for a laugh. It should make 500hp easily enough which, with a manual gearbox, should be quite good fun. With it being a V6, there is loads of space up front for an large intercooler. Cheapest swap of the low - I bought an entire 2.7TT Allroad for £700 so I think I'll be <£2K all in for the swap, then another £1-2K for tuning (mostly turbos) to get 500hp.
The C5 RS6 engine has been done but the biggest problem with that is intercoolers. There is no space up front with that so they have to go under the headlights. Lots of fabrication, and still only good for 500-600hp, but a bit explodey at that power output. Lots and lots of packaging issues getting all that hardware in the D2 engine bay which is considerably smaller than the C5 because the aluminium superstructure is much bulkier.
RS6 engines are very expensive for what they are because there are so few of them.
The V10 I am pretty sure won't fit but I'd love to have a crack at it. The drivetrain geometry of the D3 is quite different to the D2 to fit that monster lump in there. The D2 wouldn't have space for the radiator!
RS3 5-pot would be an awesome swap. 5-600hp, 5-cylinder classic Audi sound, inline engine so loads of space at the sides for turbo and manifolds. Length would still be an issue for an intercooler so it would need side mounts too. Still quite an expensive engine since it's quite new.
D4 4.0TT - now we're talking! Capable of stupid power (800+), nice and short, bolts up to the manual gearbox, charge-cooled so no issues with intercoolers. The big challenge here is electronics - it will take a lot of effort to make it run without the rest of the car, but people are working on it. Realistically I think you're looking at a £15K swap here though. The engines are still £8-10K for the S or RS versions. Its on my watch-list if ever Prestige get one in though
I'm assuming here you'd go manual for all of them btw since the 5HP24 is fragile enough with 360hp. It would just turn to shrapnel with 600. Theoretically you could keep it with the C5 RS6, but you'd probably get bored of a £3K gearbox rebuild with every engine service
