I have considered it myself a few times.

On paper it seems the aero will be slightly better, with a nicer rear window angle, a trunk to fill the void behind the window and reduce the vacuum there and if a 4 door, smaller side windows open to the wind stream.

Theory is great and all, but has anyone ever run across empirical evidence to support the Jetta being more aero than the Golf?

How about effectiveness of the GLI rear lip spoiler? If there is still somewhat attached airflow there, the lip hanging of the back may actually reduce drag a teeny tiny bit (like .1% or something), but my bet is that this close to the rear window it is not really effective at all - but pretty cool looking. Anyone have or seen data on this?

Maybe I should start putting one together and run the hatch and sedan with the same drivetrain at RA to get some data...