At that high a mileage, I'd replace the water pump as a matter of course and chalk it up to preventive maintenance. Just one less thing to worry about on a long trip. I'm not sure if the rotor can erode enough to diminish output, but the only way you can rule it out for sure is to pull the pump and measure the clearances. At that point you might as well replace the pump. After 325K mi., I think you've gotten your money's worth out of it. If it ain't broke, and it's seen a lot of miles, replace it before you have to fix something more expensive!