As much as I hate to say it, I do think fast typing *can* be a substantial boon to your law school grades.
Most people use a "shotgun" approach on exams to one degree or another. On a given fact pattern, some things will be very important, while other things will not. Its often hard to figure out what is importaint. What ends up happening is that most people address as many issues as humanly possible in the time period. You get credit for answers the prof is looking for, you do not lose credit if you end up talking about things that turn out to be unimportant (within reason). The faster you type, the more you can address.
The people who really, really know their stuff can write less, because they have a better understanding of what should be important. However, all other things being equal, the faster typist will probably get a better grade.