A mature Shetland tup can give 21 kgs deadweight, but sending them off as hogget in their second summer I haven't got anything like that much - more like 15-16kgs. 14kgs at about 12 mo.
I cross with a variety of things. I'm breeding for fleece and for good breeding stock, but we eat the surpluses so a good carcase is highly desirable. The best two performers for me are Blue-faced Leicester and Manx.
Anything crossed with Manx seems to outperform either parent, have really nice fleece, and taste wonderful

BFL x Shetland is a really useful ewe, most of the fleeces are gorgeous, and on deadweight and conformation, the lambs perform nearly as well as any Mule - just take a bit more time

In Shetland they cross with Cheviot. Ewes much in demand to put to a terminal sire. I'd imagine this cross would finish faster than some others, and achieve really good deadweight and conformation - but I've no personal experience of the cross myself.