First of all, I am beginning to dislike the SA/disp factor. The only advantage with it is that you can scale a drawing up or down, hull, sail and all, and the SA/displacement will be constant.
The real “SA/displacement”; SA[sqm]/weight [metric tons]is rarely mentioned, much in contrast to with cars and aeroplanes. It is this SA/weight that tells about the agility of a vessel, sailing, rolling or flying.
I can follow David T’s thinking about ocean voyagers, to some degree, but the chosen SA/disp should also vary with the displacement.
On a 2.5ton boat a SA/disp=21 will mean a SA of just 39sqm.
On a 6 ton boat the same SA/disp of 21 would call for a SA=69sqm.
While a 2.5 ton Contessa 26 could easily be handled with 39sqm sail, the six-tonner with 69sqm would be more than a handful (to me, at least), unless electric halyard and sheet winches are fitted. In addition, short boats need to be pushed more to eat miles, than longer boats, so it makes sense to pile one relatively more sail on small boats.
As for “the problem” with twist and reduced performance with deeply reefed sails; my experience with deep reefing of the Johanna style sail (modified HM sails, really) is that they go remarkably well to windward. The sail can be reefed down to three panels and still retain full peaking of the yard. Moreover, the lee topping lift then seems to limit the twist to just the right one to create some camber-through-twist. Last summer I had 37 outings in my Frøken Sørensen, about half of the trips alone. Without a crew she is quite tippy and cannot set more than three panels when close-hauled in F5. This means that I get some practice in reefing and even deep reefing. Even Johanna has been sailed deeply reefed many times, and seems to thrive well. Still, I am glad for every square inch of sail I have for light weather. I guess I need full sail almost half the time in Frøken Sørensen, and half of that time I could have used more sail. BTW, her SA/disp.=24.5 - and she has no ballast...
Arne