Thanks black86glhs.
I went for a hard ride tonight, before I posted, and it looks like things may have begun to resolve.
After putting the new valve cover on, I also cleaned my throttle body and PCV hoses, and then I adjusted the idle down a tad.
Now it idles, very smoothly too, at 900 RPM in P, and the boost/vac gauge shows just a tad 'over' 20"Hg now. The gauge also moves faster now, both when I give it throttle and let off, and it gets more vac when I let off the throttle than before.
I don't understand how a new valve cover could be responsible for all of that, and I sure didn't expect this.

I am guessing the valve cover grommets were maybe leaking enough to affect manifold vac? One of them was quite a bit oily, under the metal grommet cover, and both of them were hard and inflexible.
So there is some proof the PCV baffle was restricted I think. The inside of the valve cover looked clean as a whistle though, Thankfully so did the head, cam, springs, rockers, oil return galleys (?), and the rollers didn't appear to have any scuffing at all.
I really do feel better about my engine now. Time for some boost I think.
