I had my CGEM loaded with 25Kg at a time and when it was balanced, it performed fine.. when it became slightly out of balance because I removed a DSLR from the piggy back and lost about a kilo, than it struggled a bit slewing in one direction and started to "bounce" slightly when I stopped it and re balanced the mount... so from experience I think that being close to or over the mounts limit, this is when balance is very crucial. How much you are willing to go over the limit is up to you but I think it should be reasonable.
For the record, right now my CGEM is running with a +/-35lb load on a 40lb limit mount, balanced both in RA and DEC and its smooth.