I posted the data in the BRZ thread yesterday.
GC has similar economy, similar price, and a SIGNIFICANTLY better power to weight ratio.
Weight is the only advantage BRZ has, and turning and braking are the only things it might have an advantage on... depending on the quality of the brakes, tires, and dampers.
FRS will not be just as fast. Will not cost less, and for all of that... won't get significantly better fuel economy either. That is the problem. The weight reduction also means that it is more cramped to sit in and drive, which becomes an issue the longer of a trip you take, or if your knees hit the steering wheel when using the clutch and the brakes a lot.