Don't think the issue is headroom. Take out enough cushions and you can fly any RV solo. The real issue is width, in particular with a passenger. Stuff a tall person into a shorter cockpit and the legs must splay out at the knees. Stuff two similar tall people in a side-by-side and usually the passenger has to give way, all too often maintaining the one or two leg positions that don't interfere with the copilot controls or the pilot's ability to manipulate his controls. Throw in big feet and and get a new set of problems; clearing the rudder pedals makes the above even more difficult. About all you can do is swap stick time so each guy gets his own share of leg cramps.
I'm 6-2, 34 inseam, 35 sleeves, size 13 shoes, 205 lbs. Having spent my share of time in cockeyed body positions, I chose an -8. The canopy is tall enough to use a real seat cushion, but more important, neither occupant has to share the width.....which for the pilot is about 4 inches more at the shoulders and WAY more in the knee area.