Don't know any specifics about that motor but I agree with you...generally 13.5 GPM at 4000 (pump) speed should be a bit rich for a roots blown combo. Is it a 14-71 hi helix? If so, it should still haul butt with 13.5 in it. Sounds like you may have to go through everything. I've seen two guys with the rotors swapped (both after restrips) and they will run rich and are down on power. Check and make sure that the rotor helix follows/matches the pie shaped opening on the bottom of the blower case. Is the ignition okay? How much timing in it?
For reference, my old TAD combo was 438 KB Hemi, Veney heads, R-296, 11.8/1, 14-71 hi helix, 46% over, 12 GPM at 4000, Super Mag 4 and 38 degrees of lead. Would go 5s at 230 down a dirt road. Good luck!