Motors/drivetrain are almost identical. same turbo, same turbine AR .63, same garret turbo I believe. I think that the intercooler is different though, side by side they look the same. The mjor difference is the header design is different and the heads are 3.0 heads not the GN heads. Some say they have a slightly better flow characteristic. Crossover pipe is exlusive to the TTA and the intake piping from the air filter canister is different. The Transmission was set-up a bit different to be a little more plush between shifts. Also the Downpipe is different and the exhaust is obviously different than the GN. Basically it was a specific design for the TTA to the cat, but GM used the Iroc/GTA intermediate pipe and muffler.
The main difference as far as owning/driving one IMO is that the TTA handles curves WAY better. The GN is easier to work on and modify as far as intercooler mods and there are more options on aftermarket head design.
More options for Downpipe and exhaust. Oh yeah, the TTA is a bit faster!