the best power comparison is and has always been lb/hp.
in this case:
Camaro SS Manual is 3860. auto is 3913, so I'm gonna round to 3900
3900/426hp (LS3) = 9.155, pretty good, there with the old Testarossa and Lotus Elise SC
Mustang GT Manual is 3533, so to be fair i'll round up to 3550
3550/400 = 8.875, definitely better, not by too much