I'm planning to do a very similar upgrade from RX 580 4GB, but I want at least 16 GB of VRAM and I need the PCIe slot next to the card. Even with a riser, the maximum thickness that fits is 45 mm, which leaves me with a second-hand RX 6800 reference model as the only AMD option (all the models from partners have larger coolers taking around 2.5 slots (50 mm), and lower models have less VRAM).
So I'm also glad I checked the dimensions of new cards before buying, but in my case the result wasn't that positive.. :)
Hopefully the RX 7800 XT stays under 250 W (like the RX 6800) so there is a chance for a current-gen 2-slot alternative. (The 7900 GRE is 260 W with 80 CU, so I would hope the 7800 XT with 60 CU draws less power, even if they push the clocks a bit higher... Can't wait for a more detailed leak that includes TBP. :) )