andyb - You are assuming that Eflin is the same pitcher he was last season - essentially that he was fully recovered from his off-season knee surgeries last season. That may be a valid assumption, or it may not be.
It is certainly the "cautious" assumption - e.g., that whatever benefit there was from correcting his congenital issues was fully realized in 2018, and there's no particular reason to anticipate further improvement resulting from the surgery.
But we don't know. Eflin's physical history is so unusual that we cannot assume a "typical" progression. We simply don't know - but the early returns in 2019 (yes, SSS) are...suggestive, IMHO.
One thing about Eflin vs. Velasquez that I think is pretty clear: At this point in time, Eflin is making better use of his abilities. Eflin is demonstrating that he knows "how to pitch," given his tools. Velasquez is still fighting with himself, and with his catcher. Vinnie is not in control of the mental game. Because of that, he gets lit up.
I agree that Velasquez has better stuff, and theoretically a higher ceiling. I'm not at all sure, at this point, that he has the makeup of a starting pitcher.
Maybe... maybe what Velasquez needs is to be traded to a non-contending club, for Jason Grimsley, and have some Cy Young winner read him the riot act, so perhaps he'll mature a bit as a pitcher... if you know what I mean.