I worked exhaust for sometime and built bracket racers; i haven't seen a lot of serious racers with full exhaust... just saying. a lot of it has to do with the selection of the components; a lot of people buy the cheapest headers they can find which do not scavenge well and are not conducive to making more HP.
70 Challenger RT 440-6, 73 Cuda (under const)
05 Durango, 99 Dakota, 01 Ram 4x4, 14 2500 4X4, 10 PCP Challenger RT, 01 Viper GTS ACR, 52 B3B w/330 Desoto Hemi, 70 Hemi RR, 61 Jeep FC170
9 x Challengers. AAR Cuda, 4 RR, 2 GTX, 4 Chargers, etc... (too many to list)