How big was your diffuser, when using a large drip pan I will have 450 degrees below the diffuser to get 275 to 300 near the top. That's a pretty hefty burn going and will eat up your basket of coals quicker.
Or just do what your did, smoke for 4 or 5 hrs and finish wrapped and in the oven. It does work quite well that way.
"Good judgment comes from experience, and a lot of that comes from bad judgment." -- Will Rogers