I have also had the same issue more or less since day 1, however it has never caused the engine to hiccup or stumble. When the tanks are low, sometimes one tank will show nothing in the sight tube and the other will show 1/4 tank or so. I quit worrying about it; just because one tank drains faster than the other does not mean that the slow tank is not draining at all. It still drains nearly as fast as the fast tank, which is plenty to keep the engine running at WOT.

I agree with Dorsal that 1/2 ball uncoordinated for a period of time will cause visible differences in draining rates. After I finally tweaked my rudder trim tab to fly ball centered hands and feet off, it noticeably helped balance the tank draining rates, although still nowhere near perfect.

I also think that the corkscrewing prop slipstream has a lot to do with the actual ram pressure applied to the fuel cap vent tubes. I bet if someone wanted to experiment with tweaking the angles of those vent tubes they could very nearly balance the draining rates. Bend the one on the slow tank more into the corkscrew slipstream, and bend the other away from the slipstream. Don't bother with trying various restrictor sizes in the vent tubes, because its ram pressure you are dealing with, not air flow rate.