[Dave ZS-X]

depends were abouts in the rev range your looking for gains. Janspeed dont give big top end gains the gain is all low and mid range and you cant gain torque without gaining bhp, they go hand in hand.
Nearly all would say there is a noticeable difference between having manifolds and not havingthem , just not right at the top of the revs.
You cant compare 2 cars rollered together either without knowing what power each was making as standard, 1 could have had 10 more bhp to start with !
All you can do is roller your own car before and after on the same rollers and see if its gained.
Remember we are talking nasp here though and you just dont get big gains by tweeking exhaust and induction.

Dave