different thinking
different designs
i think what happened is with the 07, they changed so much with the design, that the simplest way was to have 1 QEV. This isnt the optimal way for the ram to release spent air, but is better than no QEV, and was a decent comprimise.
For the SL74, I beleive the frame was changed enough to allow enough room under the rammer for two QEVs, so the rammer action is "better", as it exhausts unwanted air at the front and rear.
Much the same as how the SL74 has the torpedo redesigned so it doesnt stick out so much. at the time, that torpedo was the best it could be for the 07 ego. now they had time to redesign it for the SL, and they made the thing smaller.
fact is, ram / hammer based poppit guns DONT NEED qevs to work, proven by the timmy et al, as well as the etek. QEVs are a keepsake from the days of the autococker - at the time they were an eclipse-derived upgrade to increase the speed of the ram mechenism. this holds true today with the egos, but seeing as they are all fast as hell anyway, its just a nice thing to have as far as i can see it.
when i shot an ETEK, with no qevs, id never have called it a slouch in the speed department.