Actually, lump burns no hotter than briquettes. There was a test article done in Cooks Illustrated about this, I'll try to locate and post.
As Rob stated, airflow and lit fuel will control how your temps burn. If you are cooking on a WSM and want them under 300º, start out with about 10 lit coals and place them on top of your unlit coals. Leave all vents 100% open until your cooker hits 200º, then shut only the bottom 3 vents down about 75%. Temperatures will initially drop several degrees, that's normal and here's where patience comes in. Leave the cooker alone at this point, the temperature WILL slowly climb and should settle in the 250º range. 'Range' is a keyword, meaning; do not try to hold an exact temperature, because it's impossible. Let the cooker work the way it's designed and if the temps go 15º-20º higher or lower than you want it will NOT make a difference and don't fight with the vents to continuously make adjustments.