I would assume a ported LIM would make a big difference. I can't comprehend why gm would leave that much material in the runner to the gasket. The only thing I can think of is that they wanted to insure in the most underkill nature to have the air go into the heads with no interference from the edge of them. However it lookied like you could port the LIM a 1/4 of an inch all around the runners and still not worry about porting the heads to match.