Well, it would be hard to measure those differences in real life without having two almost identical aircraft, or one aircraft with interchangeable wings, to limit the variables. I actually thought those numbers for the AS.37 were not terribly impressive as it was pretty heavy (400 kg/880 lb empty weight) for what is essentially a VW-powered two-seater, so it's hard to say from that example. The payload of 220 kg/485 lb for people, fuel, and baggage is not bad for such low power, but it sounds like the plane would have benefited from a serious diet to trim the empty weight.
I wonder if putting the props on airfoil-shaped masts projecting left and right from the fuselage above the rear wing might be more effective? You could have a removable, self-contained unit of engine, mount, and a T-shaped structure with the props on the ends of the T. The wing installation would then be much simpler and folding or removable wings could be an option. Better yet, put one large prop on a single vertical mast and give it a T-tail in the propeller stream rigged to counteract the nose-down tendency with power like many amphibians.