I've checked a couple sources and the static compression ratio is listed at 10.0 to 1. That would indicate probably at least 90-91 octane fuel. Now M-Benz does use a really sensitive knock sensor to prevent engine damage but once the timing gets pulled you're gonna loose power and economy. So while you won't hurt anything using regular there won't be much money saving if the MPG drops off. You could always test the effect by filling up with regular on the outbound leg of a trip and using hi-test on the return.
Keep the cruise on and see if there is any difference in MPG. I'm willing to bet the premium fuel improves the efficiency enough to offset the additional cost...

Slim