The old DTM (90s', Merc 190 E Evo II and E30 M3 DTM) had 2,4 straight 4s with 350 BHP. RWD. So they would be quite a bit slower than GTRs. The Alfa had a V6 and AWD, Audi used a V8. Don't know about the Alfa, but the Audi had over 400 BHP but on the other hand it was 500 kg heavier (or even more).
The current DTMs are just some cars with either an Audi or Merc badge on them. 4.0 V8s, restricted (with a certain diameter pipe) to 475ish bhp, the rear end is the same for both brands (gearbox, diff, rear wing - all by the same supplier), drag coeficients must be the same i think and the engines are made by each company. THey handle a lot more like a single seater since they are made form a tubular space frame and have a carbon fibre body over it, that resembles either an A4 or a C class.
I'd prefer the old DTMs TBH -
http://www.youtube.com/watch?v=nUAE-819OG4