Sure the C6 is much stronger, but also bigger and heavier. I am pretty sure it will not fit the Pinto bellhousing we are talking about. Also I am not sure I would try to fit C6 behind the Cosworth engine and in a Sierra body. There was not much space for working at the shifter and the fittings for the cooler tubes etc on the C4.
C4 is one of the most popular transmissions in drag racing for exactly lighter cars, like the Cossie, when upgrading it with stronger kits like the TransGo. Any way, it is working just fine in this car and setup.