Some specs...
997 GT3 RSR
RWD
Weight - 1225kg
Power - 485 bhp
Torque - 321 lbs/ft
Length x width = 174.8 x 77 inches
WRC Ford Focus
AWD
Weight - 1230kg
Power - 300bhp
Torque - 406lbs/ft
Length x width = 170.1 × 70.2 inches
So for the same weight, the WRC car is torquier, smaller,and AWD, which will help it on the tighter tarmac stages, while the RSR has the higher HP for any long stretches of tarmac. Remember too that the WRC car is set up for Rally, with things like an E-brake set up to help rotate the car easily, and AWD is a tremendous advantage laying down the power out of tight corners...I guess it all depends which rally stage you are racing...
As for Porsche in the WRC, they won most recently the Monte-Carlo Rally in 1978 with Jean-Pierre Nicolas, and had been competitive in rally racing around the world until AWD became required to win. In some rally's even today, Porsches compete and win, but not in WRC.
