I have the same problem. Pretty much same setup but I use the ique, so I'm pretty sure I get enough air in. My uds loves 250*. I can get it to hold 325* but it takes a while to get there. The fastest way is to remove the lid and let the fire build for a while, and I use a cheap pizza pan diffuser.
I have just accepted the fact that the majority of my cooking will be 250*-275*. I use my kettle for anything hotter.