RE: 6.5 sail for gusty lake sailing
This is a pretty "tough to answer" question.
If I was going to buy a "next size smaller" than the 5.5 Retro, it would be the 4.8 Hucker.
I can't really give you "first hand" advice on the Hucker 4.2m2 as I haven't sailed the one I have yet.
The 6.6, 5.6, and 4.8 Huckers have been my "go to" sails for the last several months. There were a couple of days (before I had the Hucker 4.2 m2) where I used a 4.2 Revo as the 4.8 Hucker was just a bit too much.
I do not have a 4.5 Retro (been using Revos and Huckers below 5.0 m2) for the last couple of years so I'm not able to make a "first hand" comparison.
If you have any smaller rigs (in the 3.8-4.2 m2 range) then I'd suggest the 4.8 Hucker.
If the 4.5 Retro was going to be your smallest sail, then either the 4.5 Retro or the 4.2 Hucker would pretty much cover the same range.
If you have a selection of 400-430 std. and RDM masts, you can tune the 4.2 Hucker through a huge range. If you want power and a tight leech use a 400 std. dia. mast. If you want power with a little looser leech, use an RDM 400.
The 4.5 Retro needs a 400 Std. dia. mast. Not sure how the little Retro would work on an RDM.
I think you can easily "juice up" the 4.2 Hucker to give you about the same power as the 4.5 Retro, and you can probably tune the Hucker down almost to 3.8-4.0 with an RDM and lots of downhaul.
So, the sales guy is not very far off the mark, but I would agree with you that 5.5 m2 Retro to 4.2m2 Hucker is too large a gap.
I'd suggest the 4.8 Hucker first, and if you really like it (I'm reasonably sure you will as it suits your B&J Freeride sailing almost better than the 4.5 Retro) then look at the 4.2 m2 Hucker.
Hope this helps,