Penalties of PCIe generations? (2x R9700)

I just bought the GPUs after deliberating and debating for over two years. With costs not coming down any time soon and me just wanting to get this massive todo-box ticked, I decided to just YOLO it; the GPUs are the most volatile, followed by RAM, rest seems more or less stable.

But actually, RAM is one of the reasons I am unsure about wether to chose a SP4, 5 or 6 based board. I am most familiar with AMD CPUs, so that is where my tendencies lie. Unfortunately, RDIMMS are going to absolutely undress me... x.x

However, if I could stick to a DDR4 / PCIe Gen4 setup, that would save a pretty penny. Now I do not intend to offload to system memory, but even a small, single-stick of DDR5 RDIMM is stupid expensive - DDR4 is fine.

The question is: What is the penalty of PCIe Gen 4 versus 5 in regards to inference? I will be using llama.cpp with ROCm, fronted by llama-swap, utilizing both GPUs for inference and VRAM pooling (so, 64GB in total).

Thanks! =)