It doesn't force you to hit the ball way out to the left, what it does do is force you to rotate your torso further and faster, which pulls your shoulder forward further and faster, which gives you an even lower racquet drop, and which also loads more energy into the ssc system which allows you to internally rotate your shoulder a lot faster.
What Sampras's move does is reduce the amount of lateral movement necessary in order for the racquet to come over the top. This does two things. First, it means more racquet speed will go into pace and topspin, than sidespin. It also means that the movements related to pronation occur earlier in the service motion, which increases the racquet speed into the ball. The downside is that it sets your initial hip position at a unusual angle, which is maybe not what you want when you're reconstructing the serve.
I'm not being disingenuous here Tricky, I really and truly amn't, but I have no idea what you mean by leading with the elbow.
Basic arm motions involved with swing sports (esp. true in baseball nowadays) go through stages of abbreviation. First, the movement is traced or led by the hand, which is necessary to code into your muscle memory the basic shape and coordination of the movement. Then, in most arm related motions, there's usually one stage of abbreviation where then the movement is traced or led by the elbow, which is also associated with having a "loose" or "relaxed" grip. In some motions, there's one more stage of abbreviation, where now the movement is traced or led by the shoulder.
The rationale for these stages isn't really for the sake of having a tighter stroke; it's that it enables the shoulder to rotate more freely. That is why abbreviation steps naturally lead to lower racquet drops; without the impingement of guided/tracked elbow deviation movements, the shoulder muscles can properly stretch and contract with much less restriction, thereby releasing more energy.
but it's the kinetic chain that which is responsible for loading the reflex action, which is what gives you more racquet head speed.Your post seems to agree with me on this point although your initial statement doesn't.
Where I disagree is the implication that this leads to a direct casual effect for racquet speed. If the racquet drop is executed incorrectly, you can execute the hip rotation and leg drive perfectly, but little of it will load into the shoulder. That is the underlying argument for the "learn arm motion first, worry about leg drive later" stance. It's not so much that leg drive "only contributes" another 10-20mph from the serve -- it's that leg drive is only one part of the kinetic chain and has less influence over the racquet speed than the shape of the backswing/takeback, level of abbreviation, and above all, a well executed proper body sequence of the racquet drop. It's considered only after the arm motion is nailed down.