A high protein diet will result in very large lambs if fed in large quantities. It will also result in more birth problems, and possibly a less than ideal start for the lambs. Hence the guidelines of recommended maximum amounts to feed ewes at each stage of pregnancy.
However, the very best food for growing good lambs is, as mentioned by Buttermilk, a plentiful supply of good clean spring grass. It is high in energy and protein and available ad lib 24/7