I run a medium I got with an 086, Ive ran it on a picco.28,mach.26, os VZB vspec,GS B03,jammin .21 and Go tech 7port turbo(08).I find it helps engines get on the pipe quicker, not so much increases bottom end but the engine will spool into its powerband quicker.
The only engines ive used it on that i liked a round header better was the two three port .21s