The problem with that logic is you can teach a guy to catch the ball but your guy will never get any faster and with age will slow down ever more. Look at what Proehl did Ginn for example. If he stayed here he would have improved even more.
Separation can be achieved by running routes correctly and timing them right. Speed isn't everything. Look at Greg Olsen. He's a perfect example. He's about average speed. He's not a burner. But he's one heck of a route runner and has amazing hands. Another example is down in Miami. Wallace versus Landry. I'd take Landry over Wallace any day of the week.
So if you had a chance at Deandre Hopkins or Sammie Watkins who do you take..
Both are speedy receivers though. Between those two I'd roll with Hopkins.