When you Weld at 300 Amps or more you will need at least a nr. 6 Cup at 7-8 Liters per Minute.
When i am in the 300-400 Amp Range i often go to a Nr. 7 because the Tungsten get so big.
The arc Force and the puddle size at this high amps are much bigger then on 20 Amps so you Need to make shure you got enough coverage.
Maybe Brad can Tell you more, he also has a 400 Amp Machine