IMHO, the NACA 6-series airfoil that the Sonex uses is not like the airfoils used by Cessnas or many of the STOL aircraft out there. Therefore it is less-likely to receive much of a benefit from VGs (or, at least less of a benefit than many airplanes).
Here are a few articles for reading:
https://en.wikipedia.org/wiki/NACA_airfoil#6-series http://web.stanford.edu/~cantwell/AA200_Course_Material/The%20NACA%20airfoil%20series.pdfVortex Generators (VGs) are designed to do two main things: (1) Avoid separation bubbles where the airflow detaches from the wing, and (2) Force a transition to turbulent airflow. While laminar airfoils _can_ have separation bubbles, putting VGs on - forcing turbulent flow all the time at that location - has the potential to reduce performance or cause as many problems as it solves; if you're taking laminar sections and turning them into turbulent flow (with higher associated drag). I think this may be especially true given that the NACA 6-series airfoils were designed to provide low drag via laminar flow at typical cruising speeds/lift-coefficients. In this case, the VGs are essentially working against one of the main design factors behind the airfoil!
Besides - if you want a STOL aircraft, there are plenty of fine kit aircraft out there that will do the job better than the Sonex ever will. The Sonex has a role, and it performs its mission just fine; there's no need to jam a square peg into a round hole.
--Noel