RE: 6.5 sail for gusty lake sailing
I have 4 Retros - 10.0, 8.0, 6.5 and 5.5. I have sailed the 10. and 8.0 for a couple of years and love them. The 6.5 and 5.5 are newer and I have sailed them about 3 times each. I like them too. It is so nice to have all four sails rig the same way and behave the same on the water. They all work well for me in light and strong wind.
I rig them on 75 % carbon masts that are the recommeded sizes. I have new 430 and 520 Maui Sails masts, a 490 Powerex Zfree and a 460 Joystick. I only use an adjustable outhaul on the 10.0 cause I don't usually need it on the smaller sails.
I think you would be very happy with the 6.5 Retro since you are with your 11.0.