RE: 6.5 sail for gusty lake sailing
I think the 6.5 Retro will certainly "do the job" for what you seem to want.
Easy to rig, huge tuning range, lot's of low end power for it's size.
Are there any other sailors on small Retros at your gusty lake?
Bruce P. designs and tests the Retro's in the Columbia River Gorge and that's a very windy but also very gusty place for sure.
Get the Retro, and then see if the guys that weigh the same as you do are faster on their similar sized cammed sails?
I don't think you will see many of them, if any, passing you on the 6.5 Retro.
Only thing I'm wondering about is the NP X9 proto mast.
I'd suggest getting the sail, rigging it on your existing mast, and if it doesn't look right or seems to be lacking in any way, try it on a Sailworks or Powerex mast and see if that doesn't fix any problems.
If you have a mast compatability problem, you can get the correct mast after you've determined that there's a problem with the X9 mast.
Hope this helps,