As we suspected, the performance of the two units is nearly identical since they are based on the same design. The only difference is that the 12V line of the 1kW unit seems to weaken more when the unit is stressed at its full capacity and the unit could not handle an additional 2A on the 12V line above its 1kW rated capacity before shutting down whereas the 800W unit could handle 5A extra and run at 860W fine for several minutes, perhaps a sign that the 12V rectifier circuit in the 1kW X3 unit is being pushed to its limit. But as long as it is working fine at the full rated capacity of the unit we certainly cannot consider this being a drawback. The voltage regulation circuit of the X3 series seems to be working very well, keeping the lines considerably stable when the load changes quickly.
For everyone's information, both X3 800W and 1000W are based on Andyson's AD-800 platform, which natively is an 800W PSU. That's not to say that the 1000W isn't designed to do 1000W, but the wheels are going to start to wobble a little going from 800W and up on that unit.