I personally think we should resist the temptation to separate out values from the discussion. Like, even if it was the case that some extremely high-level of inequality turned out to be "optimal" in some GDP-maximizing way, the question still ought to hinge on some notion of fairness or justice (in my view).

(Although, I guess if your metric is "citizen satisfaction" maybe that's not as terrible).

In any case, there are lots of case studies showing what happens at the extreme inequality end of the spectrum. As I mentioned in another comment, my favorites come from Piketty "Capital in the 21st Century" and Acemoglu & Robinson "Why Nations Fail".

In the latter, the case study on the rise and fall of Venice is particularly fascinating - huge economic growth due to inclusive economic institutions that promoted social mobility, followed by a downturn once the aristocracy moved to entrench their own interests at society's expense. This seems to be the central thesis of the book, although I'm only a few chapters in.

The parallels with modern US politics are pretty hard to ignore though.