I have a lincoln 3200HD, and run 0.35 flux core through it.
Argon & smaller wire won't penetrate as well.
With the heat all the way up, and wire speed around 30%, mine makes good penetration in 1/4". It's rated to 5/16ths.
It has put brackets on both an 8.8 and a hp30, built half my control arms, my steering & trackbar, frame side mounts, front bumper, and my roll cage.