I currently have an iOptron CEM25 mount and I will be using a EdgeHD 8 and guide scope with a small guide camera and a full frame DSLR. Total weight will be approximately 19.5 lbs for the payload supported by the mount. The CEM25 is rated for a payload of 27 lbs and so my planned payload is 70% of the mount capacity.
I am considering getting a iOptron GEM45EC to get a max payload of 45 lbs, or 22 lbs payload capacity for astrophotography, if it will be up to the job. The CEM70EC costs only $650 more but the head weighs nearly twice as much at 30 lbs. Moving the mount and tripod in and out of the house would be a bit of a pain with the CEM70.
I know about the "rule of thumb" with regard to reducing max payload by 50% for astrophotography but I wonder if this has been verified empirically by anyone in a controlled manner by gradually increasing load and then reviewing the resulting images. I should have a well centered load with the SCT and little leveraging of the load or torsional flexing as might be the case with a 100mm refractor telescope.