10-20% LOL?
If we exclude RT the 7900XTX in 4K is 2% faster on average compared to 4080.
If a 4080 can fit in the NZXT H500 it can fit in the overwhelming majority of pc case. All the non reference designs have similar size regardless if it is 4080 or 7900XTX, so no, the size is not an argument.
From Techpowerup 4K Rasterization comparison 7900XTX vs 4080:
Hitman 3: 6.5% faster
Dying Light 2: 7.3% faster
Divinity: Original Sin II: 7.6% faster
Cyberpunk 2077: 8.3% faster
Assassin's Creed Valhalla: 9.4% faster
Battlefield V: 13.9% faster
Watch Dogs: Legion: 15.3% faster
Red Dead Redemption 2: 17.7% faster
Resident Evil: Village: 18.2% faster
Far Cry 6: 21% faster
As for the physical size of of the 4080 play it down as a non-issue in your headcanon all your want, but it reality is that its size
IS something which the users would have to think about when it come to purchasing decision.
- Nvidia Founders Edition: 304 x 137 x 61mm (3-slot)
- ASUS ROG Strix – 357.6 x 149.3 x 70.1mm (4-slot)
- ASUS TUF Gaming – 348.2 x 150 x 72.6 mm (4-slot)
- Colorful iGame Neptune – Card: 253.5 x 170.8 x 41.5mm (2-slot), Radiator: 394 x 119.2 x 54.4mm
- Gainward Phantom GS – 329 x 142 x 70mm (4-slot)
- Galax SG/ KFA2 SG – 352 x 153 x 66mm (3.5-slot)
- MSI Gaming X Trio – 337 x 140 x 67 mm (3.5-slot)
- MSI Suprim X – 336 x 142 x 78 mm (4.5-slot)
- MSI VENTUS 3X – 322 x 136 x 63mm (3-slot)
- PNY XLR8 Gaming Verto Epic-X RGB – 331.7 x 136.9 x 71.1mm (4-slot)
- Zotac AMP Extreme AIRO – 355.5 x 149.6 x 72.1mm (4-slot)
You simply can not generalise and expect everyone to have a full-size/over-sized ATX cases; there are plenty of people with Micro-ATX PC which the length of the card might not necessarily be the issue, but the card being 3-4 slots in height and/or 140-150mm in width is (side-panel clearance issue). Like it or not, the reality is that oversized card will alienate some userbase, and the simply reality is smaller cards WILL fit in more cases than a larger ones- that's just how physics is.