That's an interesting observation, one that I've never considered. Could the change in highligts be seen by printing a step wedge?
The issue is that if you get too much pressure, with some humidity in the sensitized material, the negative may stick to the sensitized material. This will of course damage the negative, and yes, probably cause show up in the highlights.
When I first bought my NuArc 26 1k (used of course) the vacuum gauge read 13. Two days ago it drastically dropped to 6. I called Mr. Print and they sent me a rebuild kit that consisted of a rubber diaphram. I just got done with the big rebuild job 20 min. ago. It took a whole ten min. to do. I replaced the diaphram and it is back up to 13 on the gauge. The old diaphram had a tear in it. So I think you are OK with 15 " of merc. You may want to just hit your coated paper on both sides with the hair dryer a little before you print and chase any moisture out so you don't ruin any negatives