zeric, perhaps others will provide some input. I tested on APW7 and APW3s, both at 240V. I also considered the "80 percent" rule -- in fact, higher overclocks, if possible, may require the APW7 as you have pointed out. In my particular environment I monitored equipment with a thermal imaging camera and never found the APW3 to be outside of what I would have been worried about.
Are you experiencing problems, or just a question of concern? Regardless, I think this is an excellent question.
Jason
As we talked about on dicord I'm not experiencing problems since my z9s are still on their way to me However, I've read several places that 85% should be ok as well - if you have sufficient airflow. 1600 x 0,85 = 1360.
On paper it should be ok for 650 mhz, but more would probably be too much.