I haven't been out to that dust collection website for a while, but one
thing I do remember is that he said most DC's, including the one we're
talking about, have max efficiency with a five-inch pipe. A six-inch
opening is too big, and if I remember correctly you end up with the
same result as you would with four-inch.
I could be wrong about that, so if anyone feels like jumping in and
yelling "BUZZZZZ! WRONG! WRONG! WRONG!", feel free.
If you're into that sort of thing.