It's a 100% racing car.
They use the shell of a road car, but even that shell is heavily modified.
Kind of like a NASCAR.
http://www.youtube.com/watch?v=tMsOOwGzk1E
I think the 155DTM does not display exhaust flames because of the direction it should be spraying the flames.
I think, simply put, that PD can make flames appear in a horizontal drawing plane, but not in a vertical drawing plane as it would require with the 155DTM.
Making an exception for 1 car would take too much time?
Also a little DTM note:
'Back in the days' it was Mercedes vs Alfa. Super close racing, cars were still made of metal, people could still wheelbang without getting some sort of stupid drive through penalty..
What the Mercedes drivers would do is: bump the Alfa's in their rear end.
Mercedes even made a special front bumper for this purpose.
The aim of bumping in the the rear of the Alfa's a bit, was to bump hard enough to actually close off the exhaust, you know like squashing a metal can

With the exhaust rammed shut, or at least 'made smaller openings' the Alfa's would loose engine power hence get slower.
Alfa responded by making an undertray sticking waaayyy out of the back of the Alfa, so that Mercedes' trying to push the exhaust closed would hit the metal bars under the Alfa, ripping the front tires of the Mercedes
Hahaha good times, good times.
Now they get a drive through for even looking to eachother in the wrong way.
Bah
Golden years:
http://www.youtube.com/watch?v=Gw-AxiVqihM
*also note how they drive over the Nordschleife