Interpolating the values actually is an interesting idea, I'm gonna keep that in mind for when I finally get some time to take a shot at my wiimote library plans. But since reading out the wiimote data seems to be the limiting factor in the OP's application, the best thing to try first probably is to move it to another thread. Especially if you want to be able to read out multiple wiimotes at interactive rates, one thread per wiimote might be worth considering.

The transfer speed seems a bit low though. Assuming reading the wiimote is indeed the limiting factor in your game, this would mean the IR data only comes in at some 20-25 updates per second. This probably will result in some choppy movement, so I'd image the wiimote itself is designed to support a higher transfer rate. Is there still room for optimization on reading the data? If not, interpolation definitely will help you out.