The guys who look for every edge possible are the Control Line Speed flyers and the RC Pylon Racers.
These events have been going longer than most people have been alive. Swept wings have been tried, but for models it's more about "SHOW" than it is "GO".
One of the most popular .049 powered models was a swept design, called The GLH. I scaled the GLH up a little bit for a hot .12 engine and without doing a side by side comparison with a more conventional winged plane, having a swept wing on a model this small doesn't seem to be a handicap.
A fairly popular mid-sized RC swept wing model called The Weston Magnum has done 196 mph with the same engine / prop that RC pylon racers with more conventional wings are doing about the same speed.
It's easier to build the conventional wings lighter and stronger, so the advantage of un-swept wings will show up at some point as the models get larger.
Choosing a thinner airfoil, like what they use in Q40 racing will help. Within certain limits, shedding wing area doesn't pay off as well as using the best airfoil. Real thin wings are trickier to build, but there are a few really good threads in the racing forum that take you through it step by step.