I just test fitted the bearing cap and there appears to be no play at all, but my estimations may be inaccurate. There's a rubber ring in there that might absorb any vibrations, but I could be wrong. The threads are something like 75%-90% tapped, I would imagine. So that would leave 10-15 thousandths of an inch off the total diamater of the tube, if my estimation is right. I don't wanna take a chance unless you're sure that I wouldn't be able to tell and it is indeed affecting my bearings. I don't know a lot about different headset designs, but I can say that the bearing cap (and rubber ring) is the only thing making contact with steerer, and I'm not exactly sure if the vibrations would be as damaging to the bearings as they would be to the cap (or both?). Maybe it has to be installed and tensioned for me to even tell if there is play.

Oh, here's a diagram of the headset. The steerer does not make direct contact with the bearings.