I betcha if you add up the area of the seemingly small round holes in the gasket and compare that to the amount of area in the outlets in the top of the head, the outlets are the constriction.
I can testify to that! My bone stock 2001 125SX, which has had five sets of tires yet still on it's original clutch and top end, recently smoked by buddy's '06 YZ with a fresh 144cc top end, pipe and all that jazz, in a drag race. My 125SX has seen hundreds of hours of track time and still hauls ...