You also had SLP doing FireHawks.
I guess the closest you get now are Saleen, Roush, Shelby and Henessy but they are actually considered manufacturers if i am not mistaken. I know Saleen and Shelby are. Does Callaway still do stuff... i guess they do:
Callaway develops and installs powerfully engineered packages for GM vehicles providing significantly increased performance and value.
www.callawaycars.com
OH there is the Bison, that is probably closest to the arraignment that was in force for the Grand Prix and Syclone (i think Mercury owns Cyclone)