The pellet trained LBM that I put in this Spring are (well, were 'till the temps cooled off) eating the AM600 and they are 12"-13" long. I caught one and it was really chunky in comparison to the other LMB in the pond. Hopefully next year they'll graduate to the AM LMB food.
FWIW, I stocked some trout about a month ago. They are spread out around the pond, and not all of them are eating the pellets. The ones that are have grown about 20% bigger than the ones that aren't eating the pellets. (AM600)