It would be even more effective if you put on some weight---maybe 50 pounds or so. I can envision a stringbean throwing a baseball, and I don't really think you want to do that, so you need to eat more---a lot more, good healthy protein and carbohydrates, and get to the gym and work on building up some muscle. And you really need to get some good speed on some of your pitches, so that when you do change speeds the difference would be amazing.
I had an incredible pitching coach who told me some things about changing speeds. He said, "Move the ball around---high, low, inside, outside, work the corners, change speeds---and stay away from the middle of the plate." A lot has to do with the grip, how you hold the ball, and your release point. I don't know what your arm slot is---your arm angle---but whatever it is, you can do a lot more with it when you gain more weight and more strength. And a really good pitching coach can help. :baseballpitcher: