For me diminishing returns start when there is sound with enough headroom. After that it's just about personal preferences.
I haven't tried an IEM more expensive than my Hybrid 3 Pro (700€), nor do i have an urge to. I started my journey with the intention to find "The One" I'd be satisfied with and could whole heartedly enjoy my entire music library with. I fell in love with the H3 pro and I knew I'd found the one I was looking for. Then i set out on a quest to find something with similar tonal quality for a lower price to chuck in to my backbag. After reading and watching a bunch of reviews I decided to go with the final A4000. It's tonally close enough to the H3 pro so that there is no "tonal disconnect" between my setups, just pure enjoyment without my brain having to adjust or "correct" anything. I got the A4000 for about 130€ and the H3 pro is 700€, and I'd still buy it if I already had the A4000 before it. It's just that much better for
me and
my budget to justify 5x the price
Now I have a bunch of stuff I don't listen to, some of it I gave to my wife but some are just gathering dust
Tho I don't regret any of those purchases either because I purposefully got gear that graphs differently and with different driver configs to find what I like. I hope this didn't go too far from the subject in question.