Really, wow the welds are over 1/3 of an inch thick... that's a good find
How the hell did you notice that and ever think it would make a difference. Wonder why Mazda did that, other than maybe carelessness.

That being said, I mean really could you feel the difference? I feel the difference if I have the silencers in the dual n1, you lose a million HP because they are so restrictive.
My guess is if you did this welding mod to someones car without telling them, they wouldn't notice. For the common person who cant weld on their own I doubt it's worth 150 labor to do this mod. If you have a low mileage cat that youll keep on there and can weld on your own then seems awesome. May be worth it for him since he's already got it off the car.

I could feel a difference, but I'm pretty tuned in to my car.

The first cat was welded by my neighbor. It cost me a few beers. I did the grinding.

The second was done by a muffler shop. It didnt cost much. They did most of the grinding, and I smoothed it out. Both times I removed the cat myself.

Its a great mod for those of us who want to keep the oem cat, or have to for smog reasons.

Late follow up on this. I totally have an exhaust leak. Car flooded when I tried to start it up for the first time this spring. I ended up resorting to WD-40 in the UIM port to get it going again, which worked like a charm but got a decent smokescreen going out the tailpipe. While I was letting it burn off I noticed I had the same white smoke coming out the open hood by the turbos. It was light enough that I wasn't able to pinpoint the source, but I guess I can throw the car on jack stands and spray some more WD or seafoam and see what I see.

Is a mild exhaust leak going to make a big difference in sound?

Also, am I correct in guessing that open element filters plus an engine bay exhaust leak will make for higher than normal IATs?

For sure yes.