So, most members here wants new 4.0 TFSI in RS5...
OK, let's look at potential numbers for 3993ccm V8 biturbo engine...
465ps/6000-6500rpm(as minimun value-upper limit is at 480ps)
530Nm/1500-6000rpm
10,5:1 Kompression ratio
Engine weight about 20kg more(YES!!!) then HDZ 4.2 FSI(450ps)
Here is part of the problem-how good is 7speed S Tronic for this usage? Remember that 550Nm is upper limit. So, with tuning we could see some broken S Tronics... Next thing is turque limit for current Sport Diff... Again-about the same as for S Tronic 550Nm(just beware that Torsen quattro can handle up to 100% of power on rear wheels if needed)...
Other interesting reason is that current 7speed S Tronic is phisically too long for V8 engine. It can fit as Audi shown in some test mules but, gearbox was suffering from overheating... So, new-different 7speed S Tronic is needed(in develoment-intro 2011).
BTW, first Audi model that will get new 4.0TFSI will also use new 8speed Tiptronic with it...