Use BBQ for both searing and slow-roasting beef
In the Good Eats episode on tuna, Alton Brown mentioned that you want as hot of a fire as possible, and that you could cook directly over the chimney starter, once the coals are going:


  As for the fire, well, it's hot. It's real hot. Just take a look. It's like a jet engine down there. Now the normal thing to do would be to distribute those coals across the bottom of the grill and get to cookin'. But, um, I don't want to dissipate the heat. I don't want to spread it out. I want to keep it concentrated. I want to cook on a jet engine! And ... it's ... my jet engine so I say we'll cook on the jet engine.


He then put a grate over the top of the chimney starter, and cooked his tuna.

In your case, you could sear the outside of the roast, move it to the side, then dump the coals into your grill for a two-level fire, and  then put the meat over on the cold side.  It's not all that different from what you were already doing, but it should give you a hotter initial sear.