Thanks for that Austin... Kind of confirms my suspicions.
I think I'll just need to play around with some more prototypes. While I'm not convinced a plank is inherently better/faster for DS over a swept wing, I think the key may lie in less pitch stability ie. less elevator control input required to turn (pitch axis). Given that much of DS is turning (pitch, not roll) I think minimising drag (incresing energy retention) through turns is very important.
A plank turns (elevator control) via pitching moment of the airfoil (very sensitive) whereas a swept wing in more lift oriented (lift back towards tips relative to forward centre section) and has more built-in aerodynamic stability due to it's sweep which needs to be overcome.
However, I think there may be significant advantages and possiblities for swept wing DS models.... Give me a couple more months and you may see what I mean. Or, you may just see a new plank!