I'll assume w/ an 18 HS scouting budget, that 40 point rise in speed is not what your actually seeing. 18 should be fairly close, and I'm not sure I've ever seen a player's speed increase during his career. If so, only slightly.
But in other catagories, mhulshult is correct, 20 is very possible for a top flight HS prospect, 30 seems about peak, though you may be able to get a few points higher under the perfect scenerio, while a 40 point improvement, in any catagory seems impossible. In order to get a better understanding, simply look back at past drafts in your worlds and see how much former top HS draft picks have improved from their first ST line to their peak. Keep in mind that there will be some improvement during their first season that won't show on their player card, perhaps 2-3 points in a teachable skills (like splits or contact) between the time of the draft and the following spring training, so add those in and you'll have an idea how much a top flight HS kid can improve over the course of his career.