It doesn't particularly matter what it maxes out at if the utilisation over time is much lower. e.g. a spike loading a new texture set would potentially have to wait 4 cycles rather than 1 but if there isn't another texture set waiting to be loaded after it's a small hit on performance.

The main bottlenecks for modern graphics cards aren't usually the PCIe lanes. As I put performance tests show it's about a 20% hit on a desktop class card with current games. Before eGPUs existed similar tests were done with GPUs running on 4 and 8 lanes with similar results. The result will vary by game, texture volume and the cards memory. The more memory the card has the less it will use PCIe.

For the future Thunderbolt 5 will be 80Gb/s uni-directional or upto 120 bi-directional so it will be even less of an issue. PCIe 5 will be a similar increase but again is likely low utilisation over time.