The brand doesn't matter that much, it's the size. I use thin thread for piecing. No bigger then 50wt 2 ply and a thinner thread in the bobbin, no bigger then 60wt. If you use Connecting Threads cotton top and bottom the seam is thick. If I use a 50wt 3 ply for the top I use a 70 wt for the bobbin. Connecting Threads' Pro is 70wt, the Essential is 50wt 3 ply.
Remember the higher the wt. the thinner the size.
Mettler is not silk wrapped just silk finished.
There is a lot to learn about thread and the best place is at Superior thread website. Superior thread is very good thread.