If you are getting motion blur with that setup, I would start looking at your L-bracket, tripod, ball head and panning plate for anything that might not be tightened down. I can't tell what tripod you are using underneath all that.
[Edit] I would look at everything in your setup that is below the camera. The 24-70mm lens should easily be stable enough just connected to the camera body by the lens mount. Every additional layer/piece of hardware that you have in the support structure is a potential source of movement. You could run a process of elimination exercise (or remove everything, and start adding one at a time) to see what might be introducing any instability into your setup.