I have a 3500MB/s gen 3 1TB, 7000MB/s gen 4 2TB, 7000MB/s gen 4 8TB and a SATA 8TB, so 550MB/s.
Between them for normal use inc gaming, there's little to tell them apart if you blindfolded someone and told them which seems faster to load etc.
90% of games I have played barely breach 250MB/s read speeds when a game is loading, and in the few games that state a minimum req of a PCIe SSD, I have seen around 1500MB/s read speed give or take (Dead Space remake), which is still well below gen 3 specs.
You also don't need Direct Storage to leverage PCIe bandwidth, as evidenced by games like Dead Space remake which load the moment you click the mouse button to your save location in the game, with no further "loading" throughout the rest of the game.
Windows does not boot any faster either, and applications might load slightly faster on the faster gens due to the slight bump in random 4k reads, but again without measuring, it would all feel the same.