I can state that the ring in GT4/5/6 is coherent with the ring i see in Project CARS, which is a good sign for the accuracy of both of these versions. Im talking about track width, elevation and bumps. I think the laser scanned versions from Kunos and iRacing will be really good and will allow me to keep the driving line i use for GT and Project CARS.
I never played the FM5 Version, it may be the perception from wrong/different FOV angles as well as camera position that make the FM5 version odd. This mod version from snoopy makes me scratch my head on every single turn though.