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