I ran some tests using a 720nm converted 5DMkII.
Use of Lee resin ND's doesn't affect the exposure but their Pro-Glass ND's do reduce the exposure but slightly less than the rated density would suggest.
B&W screw-in glass filters also reduce the exposure and again, a little less than the rated density.
I guess that the resin ones only block the visible spectrum whilst the glass ones cover much of the IR spectrum as well.
Hope that helps Ricky.
Bob