When using decent quality lump, like B&B oak lump, I have no issues cruising @ over 400 with one exhaust vent, and starting with 15-20 generic store brand briquettes. I don't even have to use all 4 of my 1" intakes, 3 wide open will absolutely smoke! (pun intended).
When I use Kingsford blue, it's always a struggle to get high temps. With stubbs, I can easily cruise @ 375-400. So I guess my answer is it's not the exhaust size that's important, it's the fuel you're using.
Many folks use the KF blue in their UDS because it likes to burn at such a low temp. I couldn't stand the fact that it wouldn't get as hot as I like to cook at (275-300). Not to mention the horrible ash buildup.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)