I have 4x GC2. About 10 minutes in one stretch is the most I have run the microwave. Usually just to reheat leftovers and such. Solid 160+ amp draw while in action. As the voltage drops the amps climb. Big inverter also allows using the heat gun, sawzall, 120v compressor, circular saw, lug gun, hair drier and virtually anything that goes in a regular outlet for a reasonable time.
Your AGM does have some advantage over wet cell batteries. I think you would do ok with 300Ah. The 100 Ah minimum per 500 watts comes from the Xantrex manuals.