A few years ago, Heath Suraba with Lincoln Electric's Automation Lab helped me refine some robotic GMAW-Pulse PQR's using ER70S-3 in .045" and .052" diameters. Our main material goal was 10 gage P&O steel sheet, but we also included 16 gage. Joints were T-Joint fillet, Flare Bevel and Square CJP without backing. The goal was to determine which pulse waveform was the best to provide reliable weld size and quality at max travel speed for each material thickness and joint configuration. To provide some consideration for fit-up issues, we used a filler wire diameter width root gap in each joint.
We wound up qualifying 3mm fillets on 16 gage with .052 solid wire (90/10 gas) at 70 IPM Travel Speed. Every other joint / wire size was a bit less than that, but the average travel speed was around 50 IPM.
Now, that being said, it's a hell of a lot easier to make good welds at max parameters in a lab setting vs on a production line, but the point is the limits of what you can do are more so set by your parts fit-up / configuration and equipment capability, rather than filler metal type. It just takes diligent R&D to figure it out.
Tim