The only "GM performance eprom" for SD cars is the one you burn yourself, sorry to say. Everything else is a waste of money.
Even a custom eprom burnt by a "professional" will only do a limited amount unless they have unlimited "hands-on" access to your car....but that would cost a fortune. Eprom "done by mail or by spec" just don't work except to give you a "starting point".
If you want the "best", you need to learn to burn it yourself and learn to "tune your specific motor". This is always what's separarted the "faster cars from the slower cars" when everything else spec'd the same.
If "tuning eproms" didn't work, then the "umpires" for the IROC series and Player Challenge series wouldn't have painted a nice "blotch" over the window of the eprom to stop guys from erasing and then reprogramming.
And yes, they did "dump" the eprom from the winners and ensured the programming was the same as the "standard". | 
