No offense, Russ...I posted at about the same time you did and hadn't even read yours. I did think it was an excellent question, hoping somebody who really knew would respond. I have a couple of Junk Huskies in the pile, like a 268, 61, etc. These were NOT labelled XP, but were a high-quality saw, many pros have used them. They do have lower performance (says who?...I dunno!) finger-ported cylinders, more cheaply manufactured, as opposed to at least some of the XP saws.
I do not know if all the XP saws have closed loop transfers, but it makes sense that many probably do.
I've often wondered why E-lux didn't go with a closed-loop jug in the 50cc size to compete head-on with the Stihl 026...seems like such a natural thing to do, rather than have a plastic and an alloy model choice in that niche. We've already learned that the rotating assy is the same as the 346, so it just makes sense...
KD has posted regarding boring 372 jugs to accept 385 pistons; maybe a burnt 346 jug could be found, bored, and nickasylled to make a 351XP.