If deltas were so efficient, don't you think the full sized glider guys would be using one as the platform? They are working at about the same Reynolds number as fast RC planes. There is also a big drag penalty with the reflexed airfoils that deltas need. Swept wings and deltas work great in the transonic and supersonic range, which is far far away.
Point taken. I am not an engineer, nor do I have a degree in aerodynamics, so I will not claim that my ideas are any more than that: ideas. I would like to propose another idea to you, however. Full scqale glider guys are not trying to achieve high speed efficiency. They are trying to produce as much lift as possible while giving up very little in drag. They need to simply stay aloft as long as possible, and (except for the racing gliders) do not desire top end high speed flight. They do need to avoid induced drag during turns and acceleratied flight. Military jets focused on high speed flight (lets examine my favorite again, the SR-71) have a lift quota to fill, and do not mind so much giving up penalties in induced drag as they can easily accelerate out of slow flight, and are much more concerned with top end speed. The Blackbird was designed with a purpose in mind. It was designed to operate at 85,000 feet at Mach 3.2. It was given a wing that would fulfill this need, and nothing more. At max cruise speed it had a turning radius (at standard rate) of 100 miles. It had a LOT of induced drag in accelerated flight, and was never meant to pull more than 3Gs in a manuver, but in a straight line nothing on the planet could touch it. Had it been designed with the idea in mind of flying slowly with little or no engine power and not to have much induced drag, it would have had a high aspect ratio, long and tapered, but unswept wings. Oh! Wait- there was another fantastic plane designed like that; the U-2.
Not trying to start an argument, just throwing out ideas with legitimate historical and seemingly logical backings.
-Steve