In addition to my 7 I have an Avid MKIV with a STOL wing which uses -6 reflex on the flaperon in cruise. If I remember it made about a 3 MPH difference. On my 7, my flaps would be about -3 degrees if allowed to go completely up. Before setting a joggle to limit them in the full up position I decided to do some tests during phase I using the -3 reflex and zero positions. The difference between the two positions is negligible. I really can't determine any speed difference in cruise or stall speeds. When moving from the zero position to the -3, I can feel a slight loss of lift and compensate with a slight pitch change, but that is about it. My cruise speed may in fact be too slow to see an impact with a reflexed flap. I am in the process of finishing up the wheel fairings and will retest when complete. I don't believe the 7/8 wing would benefit significantly from the use of reflexed flaps. If you search on "reflex" there are several threads mostly RV-10 related on the subject.
Gulf Breeze, FL
N767JM RV-7 Flying