I installed two "Hall Type" vortex generators on each wing of my Thorp T-18.
Just a few inches outboard of the inner end of the aileron, and the second a foot outboard of that. What a difference in aileron control at slow speed, yet no noticable decrease in top speed. Now the arrival stall, (throttle closed) with left wing drop is less severe, and the wing can be lifted with aileron and no forward stick. Good aileron control in a deep secondary stall, level decending @ 2,000 fpm; add some throttle and the wing recouples with a whump and starts a 1000 fpm climb.
I know it's not an RV, but the point I'm offering is that just one vortex generator or two can make a big improvement in a particular aspect of control without noticable speed loss.
You can bend one up out of a piece of aluminum, and install it with some double sticky carpet tape. (use some wax remover on the wing, or it won't stay on for long)
Put it on an area of concern, like the wing that drops first in a stall. put it where you think the stall originates. Maybe you don't like the join of the flap to aileron, or think one wing tip is shaped a bit weird?
Is your wing leading edge thicker where a landing light was added? Mine is. That wing is not the one that stalls first, so the thinner wing could get the vortex generator at the thin spot. On my plane, the thin wing is on the same side I sit, so it also carries more weight.
RV-8's are sometimes seen with a vortex generator on the fuselage. Some call them strakes. They effect the airflow disturbance caused by the unique landing gear used on the 8. I haven't flown my RV-8 yet, so I can't comment on that one...