Actually, Roland Mayer of MTM took the 4.2L FSI V8 from the RS4, and put it in Roland Gumpert's Apollo, with twin turbo's, lowered compression ratio, modified pistons and piston rods, dry sump lube, and waste gates good for 650PS.

V8 BT 650 - MTM Engines

 

Just to be apples-to-apples the aforementioned UR Gallardo includes a full motor build including lower compression forged CP pistons and upgraded sleeves, as well as running water/meth injection.

That's a no brainer that any motor will handle boost with that formula whether it's the 4.2FSI or a little 1.8T.

You could get a pair of GT30's on the S5 but they would have to be remote mount like a Squiers (STS) system because there is no room in the engine bay to add any bulk to the exhaust manifolds. I've given it some thought, and that wouldn't be a bad thing, especially if you have the AWE exhaust, since you have two perfectly good heat-shielded areas not being used where the back boxes used to be.

The stock Getrag transmission should handle 600+ easily, but don't know the limits of the stock clutch/pressure plate (easy to upgrade). Don't know about what the center torsen or front/rear differentials can handle, but I would imagine that if you are not drag racing they should be fine.

 

Out of curiosity, what ever happened to this post? An S5-TT isn't something you come across everyday..

 

Not worth the trouble unless you do some extensive custom work. The APR supercharger is about to be released though.