It's going to be hard to quantify but clearly this team is by and large younger and more athletic than its competitors and that bodes well for it when it comes to winning and losing.
Team speed does not merely manifest itself in stolen bases, but in improved defense and base running in general.
As far as Mateo and McKenna go, if they were to be replaced, they would likely be replaced by guys who are also fast, if not as fast as they are, particularly in Mateo's case.
I wouldn't be shocked if Mateo was here next year as a 26th man. McKenna I would think will be gone because of the outfield crunch. However, if they end up moving Hays and/or Santander then he probably has a role on the team as a RHH OF.
So I could see both being here, and I could see both being gone. Their relative footspeed will have little to do with that decision though.