I knew I had a 13lb ISMI around and after some digging, I found it. For some reason, it passes it now. Maybe it was because the gun was new when I first tried it and now it's broken in? IDK.
So, for now I'm going to try to run the Timney with 13lb recoil and 4.5 striker springs. I'll shoot it in the off season to make sure that combo works well.