I don't know about the Witness, but the Witness Limited will use the same mags. It may take a little adjustment on the release, but I know a number of folks who shoot 10mm mags in their 40 Limited pistols.
As far as the slide, it could be a the weight or fit of the barrel, but I think if you could find a parts list it would be interesting to see if the part numbers are different.
I would certainly think a lighter spring should take care of the weight difference (if that is the difference). Somehow I think Henning would know the differences. I am pretty sure he has a 10mm Hunter and I know he has a number of Limited 40 pistols! I imagine changing the entire upper is the SAFEST bet.
Would a barrel for a Jericho 941 in 40 S&W work? I've seen these for less than US$35.