Does it even fit into a 5090 or a Ryzen 395+?

Oh, of course not, you might need up to 100GB VRAM to have those models at decent speeds even just for low-quant versions.

And all the hype about Macs with unified memory is a bit dishonest because the actual generation speed will be very bad, especially if you fill the context.

One of the things that makes Opus 4.5 special in comparison to e.g. GPT 5.2 is the fact that it doesn't have to reason for multiple minutes to make some simple changes.