I'm not sure how many users actually check the change log before upgrading. Personally I do it about 50% of the time.

Secondly, I'm not even sure how I can track the number of hours I actually put into a plugin development. I've been working on a new version of Contextual Related Posts which should be out this week, but I can't tell you that I worked X hours on this.

I don't think adding this to the Changelog would help reduce or change the support requests and/or influence donations. If a user has a question, they will ask with no regards to how much time the author put into the plugin or theme.