I have found GM owned dealers to be better with warranty claims. I dont know of a good way to find out if they are private or GM, but i stumbled across one which is and has had superior service. (Mt Kisco Chevrolet in Mt Kisco NY)

I have never had a problem with the limited mods on my car and warranty service. Hell I have had the !cags in there and complained about tranny problems with no problems from the dealer.