I agree there are diminishing returns. There are also no returns where a more expensive horn doesn't beat or isn't even as good as the horn you already have
I've found new tenor Yamaha in it's various forms, P Mauriat, even the much revered TJ Signature RAW and a Lupifaro Platinum didn't tickle my fancy over my old SA80ii. Not the same comparison as starting with a Yam 280, but my feeling of underwhelm might still be worth voicing
I was excited by R&C Two Voices, but it's a prime example of diminishing returns given that it retails a £7000
The other sax that made my heart sing was(is) Sequoia Lemon