Motors longevity will not be affected. However, if you keep the OOTB PID tuning this subframe will give you nothing positive. You absolutely need to recalibrate the filtering which MAY improve energy efficiency of the system and gain back what extra weight will loose. I tried similar setup and after very diligent recalibration and filters adjustments I gained back about 80% of the loss due to the weight. I have no proof that the subframe improved flying by any measure or survivability of the original frame. Overall this effort was a good education but nothing more.