I think stretch rears are usually foiled 50/50. At least in the set I had. This can make for slow-going.So I usually run Stretch fins in my boards, but recently I put together a quad set (Archy/fronts, QD2 flat/rears,) and am loving it, fast, responsive, drivey, with good release off the top. So are flat foil, faster, better for bigger surfers, or did I just stumble onto a set up that works really well on this particular board?
Flat foiled rears are so much faster. Futures finally released them in all three sizes. Took about 15 years!