From my personal experience, changing a bunch of stuff in the offseason means having to "start over" on a combo tune the next season. It can be frustrating, unless you are in it for the challenge. The challenge with changing several major things at once is you have a hard time understanding what made a difference good or bad. I get the urge to change it all while the car is down, but usually incremental changes are best. I have to fight that urge myself. Sometimes we are our own worst enemy....
As for bang for the buck changes, I have a hard time seeing how going from 2" to 2 1/8" headers will change much. If better fitment is the goal, then that is a different decision metric. The belt drive is cool - but you might be over estimating the utility of making easy cam degree changes. I mean, how often do you foresee doing that? (I say this as a guy with a Milodon gear drive that can also make easy cam degree changes)
I think the best bang for the buck experimentation should focus on Carburetor and Converter changes. BUT, I am a self-taught guy - and make lots of mistakes in order to learn stuff!