You are correct, the Sailworks Hucker probably is not the best
"freestyle" sail, for many of the reasons you suggest, but
as far as "early planiing power" comparing a 5.6 m2 Hucker to your
6.4 Loco isn't a very valid comparison.
Also is your "local Hucker guy" fully cognizant of the wide range of ways you
can enhance or reduce the power in the Hucker sails by using different masts?
If he was to use a soft (Powerex wave 430 RDM) mast in his 5.6 m2 Hucker, he would get lots of top twist.
Change the mast to a Sailworks 430 Backbone RDM and the top of the sail tightens up significantly.
Use the mast the sail was designed for (Sailworks 430 Joystick 430 SDM) and you get a tight leech up in the top and a very deep foil down low.
Change again to a Sailworks Backbone 460 RDM and you get a lean/mean slalom foil with a slightly loose leech up top, but still some power up there as well.
So, I don't think comparing a 6.4 Loco to a 5.6 Hucker is really valid unless you use the entire range of masts.
I think I you compared the 6.6 m2 Hucker to your 6.4 Loco the outcome would be very different.
I can rig the 6.6 Hucker to get planing at about the same threshold as the 7.5 Retro.
Hope this helps,