Frankly I find that 9 out of 10 of the six figure systems I hear (mostly at shows) make me turn tail and walk straight out of the room. And I haven't heard a system at a dealer that I rated more than "Meh" (and i work with some of the most highly regarded dealers there are) but when I get home to my system (which you can inspect in my virtual system listing) it's a completely different matter -- and also an experience I can and do constantly improve (which is what makes this hobby fun)
So what does this mean? It means that "good sound" is an intensely personal affair, and that also it's likely that we all tune our ears to appreciate a certain acoustic aesthetic. Think about the times when you've made a change and thought at first "I hate it" but once you listen through the removal of some prior tonal or temporal distortion you realize that the thing you're missing is just a coloration.
So is there a point of diminishing returns? I've not found it. But careful system balance is the key -- and simply dropping gobs of money on one or more components is not likely to give you the results you want. Careful balancing and tuning of all components, including much time and effort with the room and accessories is essential. There's one NYC based dealer who I won't name who constantly posts pictures of six figure systems installed in acoustically bare, sterile rooms - I hate to think how bad these things sound
If you can take the time to visit and listen to one of those who post here that have had long established systems (Mike Lavigne, Albert Porter etc) then you will gain an appreciation for what we are striving for ... and anyone in the Portland area is welcome to reach out to me to see if they think what I'm listening too is worth it