Yes I d[d say a 912s fitted with a c/s prop, that is what makes up any difference in the HP figures! Props have a big variance in efficiently turning torque into thrust. Using a rotax allows a bigger selection of more efficient props.
Nev, I will have to disagree about the 140hp, it really does depend on the all up wt.( and the size of the people :-) ) I have had 2 x 95kg + 1 x 135kg people (700kg auw) in my j400, off the ground in about 400m and climbing at 1000' /min and that is with the small wing and 115hp!
Tom