On Sitka Spruce
I can only go by personal experience. I "came of age" working in music stores in the early seventies. We carried Gibson, Martin, Guild, Mossman, and the like. Admittedly these were not the golden years of American guitar manufacture. I did all the set-up work so I was able to follow many individual guitars for years during that time. Not all, but many of these guitars showed noticeable improvement from year to year for the eight to ten years I was able to track them. I stand by my position on this one. Typically the guitar would retain it's brightness but add warmth and richness over time. The same was far less true for similarly constructed guitars with Cedar tops, so I concluded that the Sitka itself had an important role.
No guitar will improve much unless it is played. It is use not time that does the trick.
It may well be that an individual luthier can build with Sitka and finish with a guitar that takes full advantage of the wood's potential from the first strum. My experience is with quality factory guitars that did not have this advantage. If that is your point of perspective then I cannot give you an arguement.
Since most of us are using factory guitars I assert that those that are made of Sitka ARE likely to improve with use.