GRC is located in Mission Viejo. They prefer to use Diablo's but can also do custom chips. Anyone reading this that owns a Saleen, do not use them. My car was detonating and pinging horribly after they did the tune on my car, and had to have it redone at Powertrain Dynamics in Huntington Beach. Car is running great now.
GRC does a lot of standard Mustang's though so you may have a better experience.