I used to fly a full scale plane with full span dual slotted flaperons and felt no degradation of control whatsoever, as long as the extension was not extreme. There was some perceived falloff at maximum extension. Just remember that you are not supposed to expect the same amount of maneuverability in this phase of flight either. You should be either taking off or landing when they are deployed and not performing aerobatics.

Afterthought: They did extend the inboard portion of the flaps/flaperons considerably further than the outboard sections on the plane I flew.

http://www.bavirtual.co.uk/ops/fleet.../twinotter.htm