The Lee will make it a little cooler but easily corrected in post processing. To get really long exposures in daylight you will need a strong ND, so a screw on filter will be a problem to do composing etc. while a removable filter like the Lee or Hi-tech are easily shifted to help in composition.
+1 for the Lee "Big Stopper" filter. The cooler colorbalance is quite easily corrected by dialing in a higher color temperature.
(http://bvdwolf.home.xs4all.nl/temp/OPF/7852_S.jpg)
The filter creates a bit of mild vignetting, which I didn't correct in the above image with a TS-E 24mm. The path of the more oblique corner rays through the filter is longer, so the effect will vary with focal length.
Cheers,
Bart