The Porsche GT3 Cup car and the corresponding RS(R) variants are quite different in terms of aerodynamics, engine and suspension. The 996 RS streetcar in fact utilized suspension components being homologated for the then-appearing RSR model, if I remember correctly.

This is a different story with the RS model in the current lineup, which is more looks and numbers than the previous iteration. Since the RSR's high-strung engine would take quite some expense, there still remains the GT2 as the stronger variant, even including uprated suspension, brakes and aerodynamics.

There have been a couple Arabians ordering the previous RSR for road use, bearing in mind that Porsche didn't promote the idea themselves at all.

Engine rebuilds at the factory are nothing uncommon and not limited to Porsche cars.