I started with Dorset Downs by accident in 2003 and for several years kept them pure. However, over time the lovely ladies got heavier and less reluctant to sit on their bottoms for feet. They were calm and quiet enough though for me to roll them onto their sides when they were lying down and clip their toes that way!
After seeing a flock of 42 shetlands for sale , I bought them in to put to the DD tup. The carcasses were leaner and went to Hogg size, killing out a lot better than pure Dorset. Dorsets are the prime for Spring lamb, take them heavier and you are going to have fat.
My tiddlly flock now consists of Dorset X for the lambers and the X is mainly Shetland. I also lamb in December! The Dorset gene is strong so should come through for the early lambing ability.
I've had a 1/2 bleu du maine, 1/4 manx X, 1/4 Dorset Down lamb in January and over the years managed to get that from end of January to beginning of january.
I've lambed over the years, november - April and parted with all sheep unable to lamb December time. It comes down to what you prefer and I prefer December lambing.