try wrapping in foil or butcher paper after 4 -5 hrs next time. at 275 should be 6-8 hrs total then let it rest atleast 1 hr - 2 hrs is better.
What was your rub? I've read a cpl places that Prime is not good for Hot n Fast because of too much fat/marbling to render out- they say Prime is best for low n slow at 225* - I dunno I smoke Selects and occasional Choice and Hot n Fast has been better for me. 275*-300* around 6 hrs total.