I have an M104 3.2 coupe TT that we built almost a decade ago. It's a rather involved process including a thicker gasket, in my case a complete engine rebuild using a donor block, forged pistons, new Split second injection timing (the supplied TT one with the kit was just an adjustable screw-comedy gold!), modification of the cooling system, plumbing in the oil fed for the turbos etc, etc. It was not cheap but it does have some go and mine at .7 bar runs about 350bhp. If I was inclined changing the turbos would give the potential for yet more power but, frankly, it would overwhelm the character (and capabilities) without upgrading suspension, gearbox, brakes etc.
They are real fun and put the wind up much newer metal but to use the power on UK roads you have to select gears manually in the auto box to keep the revs up and the turbos spinning. Mine will do over 100 in third. The thing really wants its head and in this day and age could have your licence in terrible trouble pdq.
I actually think it would be better suited mated to the Getrag manual box out of the 190 2.3/2.5 but never got round to that and its not the easiest conversion I think.
Oh yes; it could also do with an LSD and some wider rims than the 8 spoke 15", 7.5" wide ones its on ha ha.
Mine looks completely standard apart from a stainless Longlife exhaust and is very much a Q car that everyone expects to be a wafty old Merc (which it is if you keep the revs below 2800 rpm).
I have far too much invested (emotionally) in mine to sell it except maybe to Jay as I know he's never had a TT and he would make her absolutely perfect.