Why did the OEs get away from the large offset wheels? I remember the Pontiacs making a big deal about their "Widetrac" Grand Prix and such. I've seen a guy with a Camaro that bought cheap Z06 wheels and changed his rear end to match the offset of the wheels. Neat trick but limits you to newer offset wheels. Is there any drawback to this?

 

Two reasons. Deep dish wheels have lots of aerodynamic drag. Wheels with the disk close to flush with the outside of the tire will have less drag. For example, look at Moon discs on Bonneville cars.

Second, deep dish wheels do not provide clearance for large disc brake calipers.

Third (NO ONE expects the Spanish Inquisition!) the greater the offset, the greater the loads on the wheel bearings. Less load means smaller, lighter bearings, means less unsprung weight, means lighter suspension parts, means better handling and better mileage.