Think you left out an important fact.
Ability of the sailor(s) to self right the boat.
I personally stop at about 15 solo on the Tiger or the I-20.
20 was great on the Hobie 17.
I've been out in 35-40 on the H-16 back when I was an idiot my first winter on a catamaran, I flew both hulls at the same time that day, was pretty scary.
I bought a wetsuit then a drysuit shortly after so I can sail year round.
Think I'd stop at 20-25 solo on the H-16 now.
It gets past the point of fun when you're thinking about how badly the sails take wear, how bad the boat could be damaged, and my all time favorite, Don't sail too far from shore in case you have to have to abandon the boat and swim for shore while your cat is blowing away.