In considering these cars, the Buick engineers made many "smart decisions" in selecting components for these cars. To let the most power get to the rear wheels.
The THM200-family transmission was originally designed for 4-cyl engines, but also had the least power consumption of any GM automatic transmission, other than the Chevy PowerGlide. It was also used behind V-6s and V-8s in the overdirve variant. It also had a 2.74 low gear, rather than the normal (approx) 2.5 low gear ratio.
The PowerMaster brake booster was probably due to the low manifold vacuum "under boost"? A neat free-standing booster that used an electric motor to move brake fluid from the reservoir into a pressure bulb to power the power brake system. Not fully understood by the normal owners of vehicles it was on (like Caprice station wagons), as if you filled the reservoir back to "Full" with the engine running, when the pressure bulb de-pressurized, the reservoir would over-flow and make a big mess nobody seemed to be able to explain. Even on the GNs, most were replaced by HydroBoost units (as GM diesel pickups used), using the power steering pump to power things.
The turbo 3.8L motors had existed prior to the GNs, but the level of execution was greatly elevated by the time the GNs happened. Not to forget the later GNX models! Total packages, for sure!
There were also some non-GN models, "Turbo T", which were similar, but a bit less hard core. Incognito GNs.
When CAR AND DRIVER clocked those really low acceleration times for their magazine artible, which got everybody's attention, it was on a winter Michigan day, so the intercooler was going its job very well.
Happy Holidays!
CBODY67