The C-Klasse DTM 2005 appears to have some issue with brake bias and brake pressure. It is very easy to lock the rear brakes and cause a spin, no matter what brake bias setting is applied. It appears this problem is exacerbated by reducing the brake pressure, which is contrary to expectations. To reproduce: load default setup, set brake bias to 65-35 and brake pressure to 80%. Accelerate to 200 kph and apply full brake without downshifting or steering. The car will spin randomly left or right once the brakes lock. It is basically impossible to make the front brakes lock before the rear brakes. Now change the brake pressure to 100% and repeat. Brakes will lock, but car will remain in a straight line and come to a stop. It would appear that the brake pressure adjustment is only applied to the front brakes, not to the rear brakes. Similar behaviour is not present in the C-Klasse DTM 2003.
Yes, the car is really difficult to handle under braking. I'm not sure whether this is intended or not.
What you're describing with the different brake pressures makes me suspect excess engine braking is the issue here. This has been on our radar for some other cars recently, so I will add this one to the list What you can try for now is to make sure you do your downshifts as late as possible, to keep the rpm low = less engine braking. It probably won't fix it completely, but might make it a bit more manageable.
He said to reproduce one could go to 200kph and then brake without downshifting at all so chances are its not just down to downshifting too early
I had the same feeling during the recent DTM 2005 @ Lausitzring DTM Short Course competition; it took me a while before being able to record a valid lap.