BTW, adding more weight to the middle of the scope will do absolutely nothing about the instability issues and just overload the mount (you also need more counterweight!)
What DOES help is to use more counterweight, so you don't have a weight sitting at the end of a long, thin stick. Getting a bigger, beefier mount saddle can also do wonders. The biggest improvement, however, often comes from upgrading the tripod. The 2" thin-walled stainless steel jobs you see on EQ-6/Atlas/CGEM class mounts simply doesn't cut it at all, when you have a 6" loaded. I upgraded to a Baader Hardwood tripod and the improvement was MASSIVE. It is, without any question, the easiest way to increase the carrying capacity of an EQ-6/Atlas/CGEM mount.