The very latest 12V-2x6 'H++' power connector on NVIDIA's latest GeForce RTX 40 SUPER series GPUs can handle up to 675W of power from the GPU.

I don't think the RTX 5090 will be as power-hungry as that, though, and I really think that the two 12V-2x6 power sockets on the AI1600T PSU are just for multi-GPU setups handling AI workloads.