Dyno'd my car yesterday, can't really factor just the intake manifold with my gains but I'm sure it helped.
with TSP 1 7/8" longtubes, SLP lid, SLP loudmouth II, Frost tune.
stock 853 heads, stock cam, ls1 intake manifold, tb, Strange S60, 4.10 gears, T56
with TSP 1 7/8" longtubes, SLP lid, SE trueduals, PRC stage 2.5 heads, MS3 cam, Fast 36 injectors, Dorman LS2 intake manifold, Warr 92mm tb, same rear end and trans. Frost canned tune (safe rich tune)