Intuitively, I know that the entire exhaust system for these vehicles are different at some point. At what point, if at all, do they become the same?

 

The center resonator. I swapped my stock GS400 exhaust system with a Tanabe HM on a GS300.

 

So, I understand this to mean that from the headers to the resonator is unique to each vehicle. Then from the resonator back to the tips is the same on both vehicles.

One more question: where is the resonator in relation to the catalytic converter? Is it before or after the cat?

 

On a GS400, it goes headers to cats to a single center resonator. This single center resonator is the common point. This in turn breaks out to left/right rear resonators and mufflers.

Being in California and wanting to stay smog legal, I considered the cats off limits. I'm sure there are at least a few on this board that have done more custom work farther upstream [cats or headers].