I don't own a S550 yet, but I have a '13 Roush Stage 3 that I bought brand new back in early 2013.
I think I put about 50 miles a month, average. Only 2,315 miles on it right now.
took a long road trip down and around SoCal (was work so was driving all over between SD and LA for a few days) and put over 1500 miles right when I bought the car. In August, I put 160 miles on the car (that will be typical) so I expect to average about 250 miles a month.
With my current car, it's anywhere from 3700-3800 miles per month due to the fact my company pays my mileage costs when I drive to a job site (doing consulting engineering).
The past two cars ('00 GT and '09 Pontiac G6) I've completely paid off through this mileage expense (including taking gas, maintenance, loss of value with added miles, etc into account). I'm looking forward to wayyyyyyyyyy less driving for work once the new mustang gets here and I go back to flying everywhere.