Our shop has used GMAW-SS for 10+ years now for 3G welds on 6 x 6 x 1/2 square tubing. Full penetration from one side. You could go 0.45 and the 90/10 mix, we use 0.35 and 75/25. You are correct, you will have to qualify short circuiting transfer under section 4, no way around it. If you can do it, you would be better advised to use pulse. In our case we often have to go through a lot of preproject gyrations with customers due to the selection of process on our part. Many of our customers have metallurgists/welding engineers in their employ that are highly suspicious of SS on heavy plate. Our company has never had an issue with a failure in the field, and its hard to imagine a failure producing a life threatening situation with our equipment, but we still encounter a lot of resistance on this point. For the most adamantly opposed, we often have to specify 100% UT in order to satisfy their objections to the process. At one point we did try FCAW, but the distortion during welding was at an unacceptable level.