just to play devils advocate, but if you have a ROUSH engine, I would believe that they tuned it well with a well matched carb to start with. I think (big guess) that perhaps the gains claimed by Pro-Systems Racing Carburators, is a generalization towards regular crate motors from Ford and GM that may not supply optimum carbs.
The only thing I can think of for a Roush carubrator, is to find out if "blueprinting" the carb will help. We do have an expert carb tuner down here in Burlingame, CA (Ollies Carburator) that may help supply an answer but IMO, if ROUSH specified the carb, then I think Patrick answered you in with an honest answer... that is to say Roush did a good job of putting the correct carb on their motor for it's intended purpose.
One final note: have you played with adding a spacer? Problem is, you'd spend a lot of money on the dyno to prove a 1" spacer added 10hp (or NOT).
Just thought I'd throw that in.