One significant advantage of using 9-10 fingers to type is that it dramatically reduces hand movement. You'll probably find that you can type longer, with less strain on your wrists, even if it doesn't improve your speed all that much.
Really, the biggest thing that the pinkies are used for are two major home row keys ('a' and 'l') and a number of uncommonly-used keys. That's probably why your speed doesn't seem to have improved by much.