I have a 2002 convertible M6 and my highway mileage has always been around 22. It was that way bone stock, after a tune bone stock, after mods, including 4.11's, and after painstakingly dialing in the VE tables from idle to 3000 RPM's.

If I drive gingerly around town I might get 15-17 mpg. Yours doesn't sound that far off compared to mine.

 

With heads and bigger cam than you in my 4.10 geared m6, I got 24.86 mpg this past spring running the a/c with a full load of luggage, myself, and another passenger. My car has only gotten under 20 ONCE, that I have checked it, and that time it achieved in the 19 range. That tank was also just out messing around, not highway miles.

For comparison, while it was lid only and stock tune, it got right at 26 on the highway the one time I checked it.