Setting the bus to PCIe5 for a PCIE4 GPU could offer theoretical performance gains.
Setting the bus to PCIe5 for a PCIE4 GPU could offer theoretical performance gains.
Setting the bus to PCIe5.0 can offer theoretical advantages over PCIe4.0 or 3.0, especially with a GPU supporting PCIe5.0. The BIOS configuration may influence performance, but it's not strictly limited by the GPU controller itself.
The advantage lies in remembering to adjust that configuration when you add a PCIe 5.0 chip later. A card rated below 4.0 will revert to the appropriate PCIe version for its connected device, while a 5.0 card in a 4.0 port will sync with the motherboard’s supported generation for both.
That setting just sets the maximum speed the link can support, if the CPU/GPU cannot support higher, it won't run at any higher. There isn't really a reason to lower the speed (outside of using a PCIe riser cable), it should default at the highest setting possible, but it's not like setting it higher will get it to run any faster. You're limited by the GPU itself.