I have played with the Hydrosport stepped 6.5M vs the unstepped but similar Solent 6.5M. My gut feel is that the stepping is more about a slightly softer ride (air cushioning) and quicker coming unstuck than raw speed - at least at the 40k level. Dag Pike says stepping only helps at 50k+.
FYI the Solent with 150 opti tops out at 44k (both ways, GPS average) in the dead calm but gets 49K (both ways, GPS average) when she is skimming the Solent chop. Both with 20l of fuel and 2 up.