Check your breaker for an indication of how high you should go. I would be surprised if you couldn't handle the full power. Just need a beefier outlet.
Discussion
it can handle the full 1420 watts. i ran it full bore for 4 months during the winter 3 years ago. also known as the winter i couldn't use my basement because it was loud and hot.
Since you know the breaker is 20 amps and the wire gauge is correct for the circuit, my unsolicited opinion is replace all the receptacles on the circuit with 20 Amp rated (NEMA 5-20R) ones. If you're running that 800w all the time, I'd recommend getting commercial grade receptacles, which you can purchase at any electrical wholesaler.