Not talking about ridge (or I think you really mean groove on the rotor).
If you can picture a vented rotor, it has 2 "plates" with the vented part between the 2 plates. In my C220, one plate is much thinner. I do not think it has anything to do with better cooling. I believe these rotors are first made with the same thickness plates and then one plate ground down in order to make it fit the application. The reason I say this is I happen to purchase discs from NAPA. The NAPA disc plates had the same thickness. So I tried putting them on. The NAPA disc fit but the inner side was hitting the disc shroud. After further investigation, I realized it was because the inner plate was thick. My point is that this might have been an after thought by MB. Furthermore, it seems to me that having an uneven plate thickness like this encourages warping more easily on the thinner plate.
I returned the NAPA discs and got ATE discs. I advised NAPA that their discs are not compatible.
I'm just not convinced that the reason the plate thickness are not the same is due to better performance. If that were true then all the rotors out there would be like so. Any thoughts???