I'd have no qualms about pushing one of these supplies to 800W. I noticed on the Habanero page that the DC input at 925MHz was less than 800W, and I doubt most people will be able to push their boards that high. You'd need a couple amps overhead for pump and fans. If you set on 900MHz and take in the expected 730W, that allows you a good 6A left over for peripheral.
Additionally, because the Habanero has independent rails, you can put 4 PSU to 3 boards if you want, without having to worry about wiring up common bus and current-share, and each one should pull less than 600W.
For the record, I'll probably be buying one or two Habaneros in the near future to play with and plan on using these PSUs. I'll likely run two kits tied and balanced for two boards, so I can use the combined overhead for cooling (my watercooling setup is very likely "overkill", the radiator fan is 1200CFM
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