on my 87 W126 the front rotors are ventilated and 28 mm thick at the beginning of their life. MB says limit at maintenance is 26 mm, wear limit is 25.4. Thus you have only about 1.3 mm wear on each side on a 28 mm disc. The rear disc is not ventilated and only 10 mm at the beginning, limit at maintenance is 8.6 mm, wear limit is 8.3 mm. I don't know your model, but this should give you an idea. Your indie should be able to give you the exact numbers for your car.
As for the running with 94 vs 98, you are right, the PO might have had the timing adjusted for running with lower octane fuel with a corresponding drop in power. But running higher octane shouldn't result in poorer running, just the same.