Well, Is a Saleen Mustang faster than:
F355, 360/challenge,430, 550, 599, 575? Probably in the 1/4, but not on a track.
Carrara, GT3/2/1 ? Maybe the Carrara, but most deffinatly not the GT cars.
Z06? 1/4 would be close... but id venture a no. On a track, not a chance.
Viper? Maybe from 0-60, but not from 50-150. Track race may be close.
Lets keep in mind, that a Saleen, is a reworked base car... so if you want to be fair, the Porsches should be RUF's, the Z06 a Lingenfelter, and the Viper probably a DLM... non of which are slower than that Mustang.