In the U.K. our equivalent of NM cable still has the ground wire one size smaller than the other conductors in all but the smallest size.
With no GFI protection, it would be down to whether the loop impedance including the resistance of the reduced ground is low enough to operate the OCPD in a short enough period of time. (That's the criterion used in Britain.)
I don't see a problem with connecting the reduced ground downstream of a GFI on the circuit. The resistance of the EGC is still going to be plenty low enough to trip the GFI on a ground fault.
