[gucom]

From a comparison of GT3 RSR and Carrera GT by Road and Track:

Where did all that time come from? Race car vs. street car, that's where. (...) As Patrick (Long, Porsche factory driver) pointed out; "Remember, this RSR is built for the track. (...) It's been dialed in for two years. The Carrera GT is the way it comes off the showroom floor, set up for a wide array of drivers and driving conditions. It's impressive that we've gotten this close with the CGT" Our VBOX on-track data bears this out. Both cars accelerate at the same rate, brake at nearly the same rate (the RSR has a slight edge), but the race car's superior overall grip translates into better performance in the corners. With its sticky racing tyres, less pitch-sensitive chassis and rear-engine layout, the RSR allows the driver to carry more speed entering turns and to get back on the throttle earlier exiting them. This produces higher corner speeds, a better launch out of turns and higher terminal verlocities on the straights. Multiply that by 10 turns on a 1,5 mile course and there's your 9 seconds.

Give the GT more mechanical grip and the story could have turned out quite differently. (...) Long was confident the Carrera GT could give the RSR a run for its money. "You lower that thing down, put a set of slicks on it and and set the chassis for more of a race trim - no material replacements other than the tyres - and on a little bigger track, you're gonna have a pretty serious fight."