I thought all 05+ Mustangs has 300/320? Also the '05s (and I believe '06s) came standard with 3.55 rears whereas '07 on came with 3.31s standard so that may offset any acceleration difference from a stock '05 and a stock '08 if there is indeed more torque in the '08s.
Taking driver ability/skill totally out of the equation, the '05-'09 should be very similar between the same rear gear and trans combos. Some cars have marginally more and some marginally less than the advertised power as well.