I seem to recall testing showing the step down from 3 to 2.5 AFTER the muffler showed little to no change in power numbers. But the 2.5 before the muffler coming out of collectors did show a material change.
As said, If you are at 500ish hp I would not be concerned about stepping down to 2.5 to get over the rear.
Or, just dump the suckers at the axles.
