TLDR; I have 2 RTX 3090s, which are PCIE 4.0. I want to get the Asus Pro-Art B850 Creator Neo motherboard, which has 2 x16 PCIE 5.0 slots; the board also supports bifurcation. What I want to do is run the 3090s in x8/x8 mode.
From what I understand, PCIE 5.0 transfers data at twice the speed of PCIE 4.0. Does this mean that if I plug the 3090s into the 5.0 slots, they'll behave and transfer data at PCIE 4.0 x16 rates? OR, will they only use 8 of the lanes since the motherboard's slots are only utilizing 8 lanes, and essentially just transfer at PCIE 4.0 x8 rates?
Alternatively, are there any better boards out there, or is just having a board that supports the x8/x8 bifurcation good? I'm running an AM5 CPU, 64GB DDR5 memory, and already bought a 1500W PSU. My case has really good cooling with a top-mount 360mm radiator for the CPU and custom 3D printed pieces to optimize airflow from the front fans to the graphics cards. I'm mostly just wondering about the PCIE 4.0/5.0 speeds in regards to this setup with the 4.0 cards.