I think the real variable is how air-tight the rig is. I know someone whose drum runs a perfect 225- with all of the intakes closed. That means air is getting in somewhere. If all else is equal, I think intake is the most critical, and why it needs to be adjustable. I have found that a 2" exhaust, such as the 2" pipe fitting found in many drums is too big. I added a 9" stack, which helped, but still had to add a baffle. If the exhaust is too big, air can enter as well as exit (especially if its windy), and the fire runs hotter. I think (and its only a guess) that the exhaust could be as small as the intake without causing trouble. Air in, air out, right?
My UDS has silicone rubber seals on toggle clamps for the intakes. They are virtually air-tight when closed. I built one with a BGE vent, but it's not as tight.
I'm very intelligent, but I mask it by doing stupid stuff.