Circular Polarizing filter and 1D Mark III?

Would a Circular polarizer effect the focus performance or speed of tracking on the 1D Mark III? I am curious is it will track as well as it does with this filter on.

It does cut down the amount of light getting in.........I'd guess 2 stops. Common sense would tell me yes.......but I can't be positive.

Thanks! It too makes sense to me, but I want to be sure.

Depending on what kind of CirPol, turned to the darkest point, about two stops and less inbetween. You don't always have to turn it to max to use its effects, you can graduate it according to need. As for tracking, shouldn't affect it much unless you are really walking the line on light. The better the CirPol, the better off you'll be in any case.