I am not sure of the gear from TheCubNut as I had never heard of them.
I had the Robart gear on my 1/4 Cub and they performed very well. A guy named Wallace used to sell Cub gear, they were supposed to be a little more scale than the Robart gear but were not as strong.
The Robart gear uses fairly thick walled chrome molly tubing. It is a very stout gear. I cannot remember how wide the stock fuse is on the Sig Cub so I do not know what the width change will do.
Good Luck!!
Anthony