Both cooling loops are identical in every way, including the evaporators, it is just the mounting to CPU/GPU that has a different mechanism. The difference in performance is due to the mechanical differences between a CPU and GPU, specifically the fact that a CPU has an IHS (Integrated Heat Spreader) and in recent years, a move to chiplet design. With a GPU, the evaporator has direct contact with the DIE and a single concentrated heat source. If the IHS on the CPU was removed and it was a single DIE design, it could achieve similar performance levels to the GPU. In theory, this would allow both loops to operate at 350W, giving a total rating of 700W for the case.
