Not a sloop, but for our cutter we looked at used sails and they were showing us sails with luffs from 44 feet to 49 feet. We were interested in a 47.8/44.5/27.6 that came off a Bristol 40′, however, we ended up buying a very cheap symmetrical and flying it like an assym just for the trip across the GOM. We used an ATN Tacker and a chute scoop. Designing a proper new asymmetrical is on the list but if you want to come check out our set up the chute scoop and tacker work great.
I attached a picture of the asymmetrical (they let us try it out) flying off the tacker and then the symmetrical with the chute scoop at the top.
As for used sails, Masthead Sailing Gear has a pretty good selection if you aren’t hung up on having a giant Vigari logo on the sail 😛
I bought an asymmetrical spinnaker many years ago and still use it. The boat won’t go downwind without it unless it blows. I have a sloop, #47. My J measurement is 15′ 10″ and I is 47′ 3″. This is the same as the tall ketch. At the time, I remember my sailmaker recommended that I go with an oversized sail because of the small rig. The width is 29’3″. You can see it on the cover of the June issue of Practical Sailor. The boat sails well with it.