A nit pick point Shoe. You are correct that increasing the Gap increases the efficiency of the wings because there is less interference for Lift. But, you are wrong as you spread the Wings on a biplane apart the interference decreases for Induced drag and thus you then have your "4" wingtip losses that are closer to 100% as the gap increases for whichever aspect ratio that you have. All of the old NACA reports show this, they weren't dumb, thus just didn't have our gagetry we have today. Think of whirling vortices on both ends of the wing. The top one interferes with the bottom one because they spin the same direction, thus the bottom wing vortex comes up and smacks the top wing vortex. Thus, for this reason a biplane with a staggered wing where the top or bottom wing is about 10-20% chord in front and about 10-20% longer has a lower induced drag and an increase in lift efficiency.

Yes, Box wing works great and decreases induced drag. A perfect winglet on a biplane is a bit harder to make as the twist angle from the top wing is opposite that of the bottom wing as they meet. But then who needs perfect?

My bro made an acrobatic bipe, weighed 17lbs with a heavy chainsaw engine and fuselage, where the wings were made of 1/4 balsa sticks for the main spars on a 78" span upper and a 72" lower. Both wings together + struts with canvas covering minus paint weight in at 1 3/4lbs if I remeber right. The paint to make the biplane look cool weighed more than the wings did together!!! Most modelers as Dick Hanson points out make their airplanes like bricks and lose the advantages of the biplane.

I would have to say that in a minimum sink competition that the monoplane is superior since they are working at VERY low speeds, Very low RE thus friction/stiction of air, since its laminar, on the surface is probably the driving factor in the competition.

Cheers
Brian