You're able to run more on less amp draw with 240. Not more wattage. Maybe I said that wrong. If you have 100amp service, that's your limit. If you have 200amp service, that's your limit ...
I save maybe 1% Watts using EVGA G2/P2 ATX PSU's on 240v vs 120v. Some Server PSU have a big difference, like 4-5%, even as far as to have a TDP limit of 1100w on 240v or max 800w if they're plugged on 120v.
As for Amps, 240v is half the amp for the same wattage, that's why using 240v allowed me twice as much rigs in an apartment setup!
30a * 0.8 * 120v = 2880w limit
30a * 0.8 * 240v = 5760w limit
Basically, 240v is safer, since the risk is of burning down is a AMPERE thing. My rig will still pull 1kW~ but on 240v its 4A~ Instead of 8A~ so the power cord is cooler and less amp go through the socket.
Exactly what he said ... That's what I was getting at.