Don't hesitate, get the 13mm. As apltx08 stated, the factory slims down right away after the outside mounting points and the 13mm aftermarket is the same thickness all the way through. It's a worthy investment. I noticed no ride difference after installing it. I did not adjust my shocks either. They said to drop your air shock pressure about 5lbs per side if you have fox floats like me. I leave mine set at 55lbs and don't bottom out the front at all, and that is with riding 2-up as well. I weigh 190 and g/f weighs 120....both weights without riding gear on.