I think that we can all agree that any prospect's reported 40 time is fake in an absolute sense. What I mean is, no Conway doesn't run the 40 in 4.5 seconds. But, 40 times are still somewhat useful to try to figure out what prospects are faster than other prospects.
If we can assume (and it's a big assumption) that in general, all 40 times are approximately as fake as any other 40 time, then we can say that a prospect with a 4.5 40 is faster than one with a 4.6 40.
Thus, I think that 40 times can be useful not to tell us absolute speed but relative speed.
As for your earlier posts, yes, you're right that as a DI prospect he should be getting better separation and that does worry me somewhat. Hopefully he can improve on that in the 1.5 years that he has left in high school.