Nice test. Good stuff. I think no gaps on corners can be an issue though if the patch panel or whatever you're butt welding in has square edges. What I mean is sometimes I'll let the metal get a little hot so the weld sinks slightly but doesn't do any warping, if I have backside access for a hammer and dolly. Then I hammer and dolly while you're presumably supposed to cool the weld. This is a quick way of doing it and ending up with straight metal, but you know that. Anyhow, when you do this that square corner CAN warp on you a tad even though nothing else warped, so I always make sure there's a slight gap on corners or maybe round it off a bit.
I had an episode on my video blog about how not to do things where I mudded a guy's quarter patch panel and it was shrunk to high hell and he didn't metal work the mangled quarter while the old metal was cut off and had access to it. I was kind of grumbling about it in the video and due to all the metal work and deciding to just peck down the highs and fill
I overlooked one thing...the squared corner warped far beyond a couple inches. The rest of the weld zone was just sunk in bad but no real warpage to deal with but that corner had a tital wave coming out of it.
I never use water but sometimes if I'm in a rush I'll let it cool naturally for about two minutes and zap it lightly with some air. Most of the time I just let it cool naturally and game plan around it. It's never really an issue with time.