I think it's more likely that a pitcher with good command/control learns a new pitch/modifies existing pitch (changeup, sweeper) and breaks out than a high velocity prospect with poor command suddenly develops better mechanics in his mid-20s. McGarry, Baker, et al., all seem to have flamed out in AA, probably b/c at that level hitters can handle FBs over the plate and they can't hit spots or get their breaking ball over consistently.
It's probably innate body control, maybe it's harder to scout, but doesn't look like something that can be coached up..