You're right, thin shells like that will result in mushy sound in deeper toms. Deep shells need thick, heavy material to force the sound into submission. It doesn't surprise McBrain has 13 mm shells - my shells are 12mm (about a half-inch) and sink in water. In a shallower shell, a thick and heavy shell would sound harsh.Yeah, the Keller shells I used were 5.4mm, and didn't do anything good with any type drumhead. I'd bet those shells you have would respond a lot better, and would suit square sizes.
That's another thing I didn't even think of. Nicko's kit was a heavy shelled sonor, something like 13mm. New shells like those I use would be less rigid.
Now THESE are deep shells: