While height is helpful to getting a fast serve in, it has little to do with the manufacturing of speed. I have hit serves in demonstrations at clubs and during workshops with radar guns standing only on my knees and hit over 85 mph (and getting the serves in) to show students that size is not the absolute key to generating fast serves that are in.
The idea of hitting serves at the top of the toss is irrelevent. Making contact at or near the top of your swing is relevent. Less than one percent of any top college players, top junior players and, of course pros, toss to the peak of their reach...with most of these top players, the lowest tosses found are still around 6 inches above the players contact point. Many players are well over a foot to two feet above contact point on the toss.
While there are a few teaching pros pontificate that the toss should be only as high as the contact point, it might serve to recognize that about the only players who practice this bit of misleading advice are those stuck at the 3.0 or 3.5 levels. The vast number of high performance teaching pros and those who teach higher levels of players teach the toss higher than contact. Obviously not several feet, but at least one foot.
Those who say it is much harder to hit a ball dropping from a higher apex than the contact point are using misleading concepts. A ball dropping from a foot above apex is falling at less than 5 mph. Heck, my 8 year old daughter has no trouble hitting a ball moving this slow. (With slice and kick no less!) Even at the 3.0 level, most players have no trouble hitting a 40 or 50 mph return of serve or volley, so why would anyone actually think that hitting a ball that is moving 5 mph be so difficult to hit? It is because such pros are totally ignorant about the dynamics of teaching an effective serve.
Ironically, a low toss is perfect for those who use an eastern forehand, waiter's grip service motion since these players typically face the net and push the serve into the service box...instead of generating not just proper racquet head speed, but also a swing path that generates the proper axis of spin.
The point of this is to remind players that using inadequate service techniques will have domino effect on the entire service motion. And as such, those who use such ineffective service patterns tend to use other elements such as a low toss, stepping through with the wrong foot, swinging with the arm, and other negative elements that contribute to poor serve results.