I purchased the Stolspeed VG's from you and tested them on the Tecnam P- 96 in our school and later on Alto's (tail dragger and trigear)
I´m Spanish dealer for Direct Fly....
With VG's we have observed a drastic reduction on the minimun speed with no noticeable reduction on cruising.
I think this is OK for both type of planes.
In the School Tecnam we gain a good margin from stall speed perfect for the school practice, and safer for solo flights.
In the Alto is good for reducing approach speed needed for the shorter span wings.
In both cases, the reduction in stall speed is around 20 Km/h. it looks very good to me!
Sure I also tested them in short take offs, and feels like adding slats....but prefer not to tell this to my student pilots... just in case they want to try.