I don't know that game speed plays a part in separation, I am dubious of that--but in terms of getting a bit closer to NFL player speeds, the tests I've seen suggest Slow speed is closer than Fast. This can change from Madden to Madden, as well, but for M18 data was pointing more toward Slow. Whether that's pleasing is more eye of the beholder. There's a number of 32-man leagues now who run on my settings, and I know for a fact some guys still prefer the feel of Normal or Fast game speed and think Slow feel sluggish. But that's just personal taste and has nothing to do really with how player speeds benchmark against the NFL. Hints of familiarity, as well--Madden historically is more of an arcade game than a simulation, and despite what some folks say they want, what they
actually want may resemble more the arcade version than the sim version.
RE: Tyreek vs Sherman, etc etc. I assure you at high thresholds, speed still matters. You line up Sherman vs a 95-97 SPD CB all day, and he's going to have his moments. But just like in the NFL, the difference isn't that Tyreek would run 10 yds past Sherman off pure speed. Tyreek is an actual good WR unlike other guys who have speed & nothing else. So the threat of his cuts, etc can get DBs to turn their hips, and boom he's off to the races. He can get huge separation at times but it's not because he just lines up & runs past them. Google press stack technique--it basically takes away vertical speed almost regardless of how fast the CB is. But it opens up everything else underneath, etc.
I just threw a couple bombs in my last league game, in fact: one to 91 spd Jaelen Strong who had just barely a step over a slightly slower DB. Another to 95 SPD Will Fuller who put a double move on a guy and had the speed to pull away when the DB hesitated. The biggest myth & straw man argument is that high threshold = speed doesn't matter. Guys in my league still put a premium on drafting speed for a reason. High threshold = speed isn't overwhelmingly the thing that matters.