the early gmt400 pellet cats arent worth crap today as far as i know, just like aftermarkets... but thats also partly due to the meth-head theft invasion. Hence the new laws/requirements for info you need to scrap a cat.
Not that i cant stand them, i just feel theres parts they coulda designed better. I still appreciate them if i see them! But there were generations where gm was just HORRID.... Lagunas and aerocoupes in specific.