Like some have said, just google hennessey and viper issues. I wouldn't let them rotate my tires but hey that is just me. Travis at boostworks or maybe LMR would get my vote for a big HP build.
John Hennessey is an absolute snake in the grass. He's been caught several times delaying production for many months, for one, but also taking parts off owners' cars and reselling them; that would be bad enough if he did it at the time of production, but he's done it many times in advance.
The deal usually goes something like this: Guy drops off Viper for supercharging/tuning/etc. John Hennessey immediately strips the car of various parts (e.g. hood, wheels) and resells them. After many, many months of no progress, owner decides to cancel his order. Oops, can't pick up the car, it has no hood or wheels!
He's been caught lying about his horsepower figures and what modifications have been done, and has generally been shady as hell. Is he a good tuner? Yes, he's built some phenomenal vehicles. However, he's a horrible businessman and I find it reprehensible that some car manufacturers are getting in bed with him.