I have a slightly shorter right leg. Rather than cleat shims -- which can cause problems -- I use a green Spenco "insole" under the regular insole. (It's flat, without any arch, and only 3-4 mm thick, so you can't really call it an "insole.") I can definitely generate more speed/power with that setup, as opposed to lowering the saddle for the short leg, and I feel more balanced when pedaling. (Try doing single-legged pedaling on a trainer, you'll see what I mean.) But you'll need shoes that aren't too tight to do that.