That's a tough one to answer, as audio is very subjective and everyone places different levels of value on items and experiences. I'll try to explain the best I can though.

Arya Stealth already sounds phenomenal, and so do the majority of headphones >$500. When you start getting into high-end audio, diminishing returns are very real. I wouldn't be able to say that Susvara sounds as much better than Arya as going from HE400 to Arya, because once you reach a certain level of sound quality, it becomes more about detail and flavor, so to speak.

The detail and sound quality are phenomenal, and is absolutely the best I have ever heard from headphones. However, if I were solely chasing better sound quality, I would've stopped at HE6se or LCD-X and called it a day. With music being so subjective and such an emotional experience, I find the Susvara to be worth it for me. I also made sure to get them at a discounted price which will ensure minimal loss if I ever decided to sell.

They're worth it to me because the improvement of little nuances in sound are the difference between thinking "this sounds really good," and "I feel this like never before, this is incredible." When you experience detail brought out in music in such a way that sounds so natural, smooth, and enjoyable, it really does make a difference. For me anyways.

If I had to put a number on it, which I don't even know how to accurately do so, I'd say from 0-100, LCD-X and Arya are an 80, with Susvara being a 90. I say 90 because I don't know whether or not there's a more enjoyable headphone that I don't know about, so I can't be certain.

Once you reach summit-fi levels of audio, it really just becomes more about finding a sound you like that reproduces music in the best way possible for your ears. It's a completely unnecessary, and honestly ridiculous amount of money to spend on headphones, but it is what it is. There are more destructive hobbies and habits for sure.

Having heard many headphones ranging from free to now $6k, I can say with a fair bit of confidence that the peak before highly diminishing returns is at around the $1k price point. So think LCD-X, Arya Stealth (on sale), Focal Clear OG, etc. Those get you top shelf sound without reaching the point of ridiculousness.

Tl;dr: They sound better, but it's hard to say "how much." To me, it's considerably more enjoyable, but to others, maybe not. Sorry for such a long response.