I've flown all over the Seatac area (even to KTIW!) My frame rate's anywhere between 21 and 50 per second, depending on who knows what. It's very stable and I only ever get a stutter immediately after changing aircraft or location. This is, however, extremely brief.
I think I'll buy it then and run it off of the external hard drive. If all you have to wait on is loading, then that's no big deal.