We don't know the performance of dev mode at this time for the Series X since MS (AFAIK anyways) hasn't told anyone the actual difference, but it's a 100% guarantee you don't have complete full access to the hardware. For the Xbox One, Dev mode only gave you access to 4 cores of the CPU, 1GB of RAM, and something like 50% of the GPU power, so it's likely somewhere around that ballpark (probably 4 cores of the CPU, I'd imagine more RAM probably, and at least 50% of the GPU). But again, you're really not going to see CFW for the Series X, especially not to the extent that you can install an OS on it, so you might as well give up on that dream. Maybe when it's EoL, but I'd doubt it even then TBH.
As for PS5, nobody knows. You might get the same circumstances as the PS4 with it's 1.76 hack, but it's literally been less than a week since it's been out so nobody has any idea. IMO I sort of doubt it, given the PS4's security was quite tight and we only really got a few exploits (and most of them on lower firmwares with no easy way to downgrade) in its life so far, but you just never know. But I can say you'd be better off buying a PS5 for exploits as opposed to a Series X, because at least the PS5 has no method to already run homebrew so it'd be actually interesting to get unsigned code running on it.