Yeah the extra weight tends to negate some of the HP advantage. OTOH the GT comes with a fairly dog rear end (3.31). I'd have to guess all else being equal the R/T would get the nod by a hair. Put some 3.73 or 4.10 gears in the GT and it might pull ahead slightly. Realistically both cars are very close and as someone mentioned it's often gonna boil down to a driver's race.

Bring on the flamaro!