Hmmm... I have not tried this bobbin case in my other featherweight. I'll try it and run it at full speed.
My bobbin tension is actually pretty tight. I normally keep my other machines bobbin tension so that when I lay it flat in my hand and lift the thread, the bobbin case stands up and just barely leaves my hand before spooling back down. On both featherweights, I have it set much tighter. I was unable to get a nice tension with it looser.
I had Mettler 50wt cotton on prior to the Aurifil. It's a smidge thicker.