The soundstage is noticeably bigger but the difference isn't huge. Maybe like 20-30% bigger? The overall listening experience for content that does not need a large soundstage is only marginally better on the HD 820 and I might actually prefer the more cozy feel of the T5p 3rd gen for audiobooks and some types of music. Imaging is much better on the HD 820 though and its easy to listen at unsafe volumes caus every time I increase the volume I hear something new in the audio. For what I was looking for (large soundstage) I got what I wanted so I wont be returning it. Though I do wonder if I should have just skipped it and put that money towards the A16.
I replaced the cable with a shorter one from Hart (the stock one is unwieldy). Comfort is better than the T5p (which is already a very comfortable headphone). I use the Dakoni fenestrated sheepskin on both, I havent really tried the stock ones. I'd say I can wear the T5p for about a movie's length without any comfort issues and the 820 for about twice as long. Its really big on my head though so it looks silly on video calls. I also own the AKG K702 and at least it doesn't give you the martian silhouette that the K702 does which is honestly peak clownish. The silhoeutte of the T5p is fine
They also included a usb drive. I dont have a sacrificial device handy where I can plug in random usb drives so I dont know whats in it. Apparently they've decided to unload some extra stress on their customers
Apologies I cant be more technical in my descriptions. I only bought these so I can watch movies more immersively so I cant speak to the shape of the sound curves and other hifi stuff that other people talk about
Overall if I had to do it over I'd think very carefully about whether I care about anything as much as I care about soundstage but for me I know the answer is no so I would make the same decision I think