Not sure what you are referring to with underpayment and overpayment, would you mind elaborating?
Yes, it would be fairly challenging to evaluate the effect of a change like this in production with a constantly changing environment, that’s why I’ve been advocating for quantitatively testing the assumption that this proposal is going to improve usability in a setting where it could be falsified. Many of the methods people have been applying in the simulation notebooks and various quantitative analyses would be adequate, but unless the outcome of an experiment has the ability to invalidate our thesis (which requires a precise enough definition of what we’re trying to achieve) we aren’t doing any science, we’re just using scientific language in order to give our opinion an aura of legitimacy.
Chaos is neither a bad nor a good thing in itself, it just imposes some barriers to our understanding. Even without bringing chaos into the picture it is easy to see that the base fee admits oscillatory solutions that might delay or prevent convergence to the equilibrium price (see the article I linked above if you’re interested), achieving the opposite of what people intuitively think this proposal is going to achieve. Hence my opinion that it would be risky to judge this proposal based on our intuition alone of what it ought to do because it is what people want it to do.
A basic economic equilibrium analysis is sufficient to realize that it cannot have much of an effect on the equilibrium price (which is IMO the most pressing problem the Ethereum community is facing). Of course that doesn’t rule out this proposal having some sort of secondary benefit. But what exactly? Will it outweigh the almost certain security costs? Will it outperform any of the cheaper and less controversial alternatives? Its counter-intuitive behavior suggests that those are all questions beyond the reach of our intuition alone, only a quantitative analysis can provide the answers.