Don't be so sure, the 6600XT clearly saturates the bus occasionally with a measurable performance drop at PCIe 3.0 x8:
The 6500XT is half the performance, but PCI 3.0 x4 is also half the bandwidth, implying that the 6500XT will very likely saturate the bus.
The performance drop caused by putting a 6500XT into a PCIe 3.0 slot could easily be the 98% drop to 93% drop in the chart above if everything scales linearly (it doesn't, but the factors that scale non-linearly might cancel each other out - we'll have to wait for real-world PCIe scaling tests like the above test to know for sure).