Going back to Dave's experiences, I'm wondering if it wasn't simply a case of too much angle on them? If you think of them as like a tiny wing, then an appropriate angle of attack will give you low pressure on one side, high pressure on the other, and a resulting vortex round the tip. But too much AoA , they stall, and the system breaks down - you don't get the pressure differential you're trying to create, and just end with an aerodynamic mess and a bunch of drag. So stalled VGs wouldn't give any benefit to the boundary layer (and would bugger it right up instead), and would certainly hurt the glide by virtue of the additional drag. Any chance you could try them again with less angle, Dave?