LAball - I would agree, only as it relates to a quick slide step, however your statement was a hypothesis based on LHP. Many LHPs do not use a slide step, except occasionally. I am needing to understand your premise more clearly. I can see where less experienced, or younger pitchers may be rushed, or not very mechanically efficient when learning to pitch from the stretch, and they may appear more fluid from the full wind up. Apart from that, I don't see what you are describing, in fact, many times I see quite the opposite. LHP aside, look at Darvish, he went strictly to the stretch..why?...because he had better control, and still could dial up 96 at will.
I posted the picture of CC above to prove a point. I have watched, and studied some pitchers that for whatever reason, intrigue me, CC is one of them. Going into front foot plant, as shown in the picture above, he is identical when you stop video from either the stretch, or full wind up. His velocity does not suffer any, from either position, and as you can see, he gets into a very powerful position prior to touch down.
If Darvish, Sabathia, Chapman, and others who either used to, or currently throw in the magical above 95+ range have no increase in velocity or control from the wind up, then no mechanical advantage is there. You have to start with the elite pitchers in the MLB, since they have the best instruction, the most gifted athletes, and have excelled at the highest level. If these elite pitchers do not show any appreciable advantage from one over the other, then one has to wonder why that is. Sure, I'll buy that an individual may have more confidence, or better "feel" from one versus the other, however actual results differing is another thing.