The intake runner length, cross sectional area, taper, plenum volume, runner volume for all the GM LS series manifolds have been designed using very sophisticated computer analysis to have a responsive engine, broad power band, etc. The FAST manifold was designed primarily by Wilson Manifolds to improve on the technology of the LS1/LS6 intake and represents a lot of technology to bring about 20 to 30 more HP at peak (compared to the LS1/LS6. Someone who is very knowledgeable about the airflow/aerodynamics of cylinder heads and intakes, can coax a few more HP out of a FAST via porting. I don't know if these statements help or not, but bottom line, trying to chop out this or that on an intake to improve flow would result in a reduction of power and drivability. Probably a significant reduction.
Steve