By shortening your bands, you will increase the elongation percentage which will increase your shots speed.
If you take the same bandset and cut them at a 400% elongation, the bands will not shoot as fast as say the same bandset cut at a 500% elongation. The more stretch in the bands, the faster they shoot. The downside is that it also decreases the band life as well. When trimming your bands, be sure to leave extra length to attach the bands to your frame.