've build several of the FT3D's, the wing on the plans is far too small. I have better results with a larger wing. To me your wing looks small, a larger wing will reduce the wing loading. Might want to consider a wider wing cord.
They seem like something they just kinda slapped on without much calculation, and if it's true that they improve the handling so much then it means the concept in general is quite useful .
The shark teeth on the Balrog seem to work quite well too, but then again, I haven't flown it without them, so maybe it's just a great plane in and of itself . I'll probably want to put either one of them on the Sbach, and the VG's would be a couple of grams lighter, so...