I did one yesterday That was about that size on my BGE and it came out very moist. I put it on at about 300 for 2 hrs, no injection, right out of the fridge, with apple chunks. Once it got to 165 I put it in an aluminum pan with a cup and ahlf of beef broth, a little woostershire and some rub. Back on until about 190 (the egg had cooled off to about 250)
rested it in foil for 90 min and sliced. I didn't trim the fat and cooked it fat side down, if that matters.
Big Green Egg hatched 6/18/10 Char-Grill gasser (rolltop storage)