I have a very similar setup, 3x4090s. Depending on the model I’m training, the GPUs use anywhere from 100-400 watts, but don’t get much slower when power limited to say, 250w. So they could power limit the 5090s if they want and get pretty decent performance most likely.
The cat loves laying/basking on it when it’s putting out 1400w in 400w mode though, so I leave it turned up most of the time! (200w for the cpu)