I've owned both in 5 series form. Power differenece is negligible in 95% of the scenarios. If you do a lot of 1/4 mile racing, then the M5C stock was slightly faster - but not much noticable until you get to triple digits. They have the same torque figures which is what gives you the sensation of "fast" until you really get on it at illegal speeds.
If you drive around with a dragy and log every run you will notice the difference in 100-200 and 60-130 numbers. But I assume that most of us are adults and don't get an 8 series to dragy it 24/7 where you would actually feel the difference
Between the M550i and the M5C, I honestly missed the fully loaded M550i more due to the rear wheel steering, much more comfortable ride in everday scenarios and and B&W. Not to mention it sounded much better than the stock M5C. Someone mentioned the AutoTopNL guy on here. He did a review between the M5C and M550i and confirmed the same. M5 was very raspy while the M550i sounded like a true v8 (more AMG like.)
Even though my car is much faster than a stock M8C, I barely ever utilize the full power. With these heavy pigs of cars I prefer having all the options over anything else. Power can be easily added and I assume most people don't track these cars because they are not really driver's cars like the M2C, M3C, etc. If speed is the biggest priority, one can just get an M3C, tune it and blow and M5/M8 out of the water for less than $2k.