Great thread and discussion going here.
I would like to add something that many may not have realized.
The effective gearing of the MT82 with 3.73's is like a T56/T6060 equipped car (GM LS's, Terminators, and GT500's) with a set of 4.88+ gears. Matter of fact 2nd gear in a MT82(2.84?) is still steeper then first in a T56(2.66). This gives these cars some brutal torque multiplication compared to other cars both old and new.
Also the difference in rpm per cruise speed for 3.31 to 3.55 is approx 200 rpm, from 3.55 to 3.73 approx 200 rpm. IMO this isn't enough difference to warrant not opting for the latter two. The minimal (if any) loss in mileage is more then made up for in the acceleration and fun factor.
As for the M6 3.73 vs tuned A6 from a 80 roll to 115.
I say it would be close since the auto will have to downshift, and the manual can stay in 4th for practically the whole race with a shift to fifth at the very end (unless the rev limiter has been bumped).
I would definetely like to try it some time though.
2012 GT 300A Lava Red Metallic, Brembo's, 3.73's, Build date 6/17/11
372 RWHP, 354 RWTQ ; GT500 AB's, MGW