There must be something wrong, at least for me. Since Patch the top Speed is the same witch or without DRS. I tested DTM 13, 15 and 16 in Monza, long straight. I can push the button, hear the sound but there is no effect.
I have tested these numerous times in both our beta-testers build and the public build of the game and have come to the same results each time since the hotfix last week... that everything is working properly. In DTM 13/14 Using DRS will get you about 10 kmh if you are by yourself going on a fairly long straight. DTM 15, I cant recall exactly, but it was between 5-10 kmh in most instances that I do recall. And DTM 16 seems to have the weakest effect of DRS, but you still get a ~5 kmh boost to your top speed, in addition to being able to accelerate slightly faster. You could try verifying the integrity of the game cache with Steam. Perhaps some of your files are corrupt. Or go to Paul Ricard 2A short, and do some testing there, and record some videos showing laps without DRS and your top speed, and laps with DRS. For the DTM 15/16 cars you will need to adjust the gear ratios so you dont top out at ~250 kmh or so. The default gearing is pretty short, but is based on hard data and feedback from Bruno Spangler, among others.