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.
Discussion
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.