a couple hp.It works great on the twins and a lot of bottom end response too it.With just reeds and the head gasket mod on my xtc 600 I was dead even with xtc 700's on the lake up to 100mph.
I did mine on a twin and it really woke it up at all rpm ranges. No rejetting needed since you are not moving more air thru the motor just squeezing it a bit harder.