I guess it all depends on who's doing the building . What's so hard to believe? A first gen Viper is only good for about 400 hp stock which is pretty pathetic for an 8.0 liter V-10.
Yeah, the Viper is good for a lot more power, but what I found won races was torque. We've still got a Comp Coupe that we ran in World Challenge and it dynos at about 650/600. The street GTS I had probably was at around 525 but like I said, the real fun for me was the torque.
Here's a shot of the three cars me and my business partner owned. The yellow one was mine, the blue one was Tom's and the one in the middle is the only one we still own.