I don't think any drives consistently hit their rated transfer speeds, which is why they always state "up to" when referring to them. It's all down to the busy state of the controller, thermals and whatnot, Sometimes my 990 hit 7000, but typically it's a bit under.
Going forwards we won't see the transfer speeds etc making a difference in games either A general PCIe SSD can load SSD optimised games like Dead Space which recommend a PCIe SSD instantly, and that game doesn't even use DirectStoage. I measured the actual transfer speeds in games like Forspoken that do have v1.0 of DS and registered read speed between 1500-2500MB/s - And those speeds equate to load times under 1 second. Now if you look at the bigger picture in future games, if DS version 1.0 is already loading in even 1 second on average and that's at 1500-2500MB/s, then is anybody going to be able to notice a game loading any faster than that?