This may not be of any particular use, but after Peter Germann's comments I was reminded of something. I have fitted a form of Gurney flap to overly slipper F3A models.
I had a model that halfway through the schedule would overspeed through a series of vertical downlines to the point that when I got a specific maneuver (I think it was cuban eight or something) I felt like the model was doing 300 miles an hour, and I had no time to think.
What started as a joke degenerated into a fixture of the model. I taped two popsicle sticks to the trailing edge of the wing, close to the fuselage. these were aligned with the trailing edge, but had the faces perpendicular to the wing centreline, so in effect, I had 3-4mm protrusions above and below the trailing edge. These were fixed (not attached to the ailerons but a fixed part of the wing). The effect was striking. Downline speed was much more predictable, and no change in trim was noticed.
After that I made a set of carbon 'popsicle sticks' and they've been fitted to the plane ever since.
With that said, from Peter's experiments and my, I believe a large trailing edge thickness may go a long way into stopping a model overspeeding, or at least creating a more constant speed, no?