Klink,
Sorry about the bad news. I don't mean to rub salt in the wound, and I'm sure I'd be spittin' mad in your shoes, too. The question I have, having read this thread through again, is what has International Diesel said/done about this? You've obviously done your homework on AMG/GEP, but there seams to be some question regarding the authenticity of the product they sell. If it is in fact a knockoff, International Diesel is where I would focus my attention.
Good Luck. I hope you get some satisfaction out of all this.
Brett
2005 GMC Sierra SLT Dually Crew, 4x4, stock
71,000 kms