40 wt thread is too thick for me for piecing. Aurifil is 50wt 2 ply which is thinner then 50wt 3 ply which most cotton threads are. There are three things you need to know about thread.
1. The higher the weight the thinner the thread
2. The more ply the thicker the thread
3. What the thread is made of.
Long staple Egyptian cotton makes the best cotton thread
Short staple cotton thread, usually grown in the US, is the cheapest cotton, mostly going to manufacturers in Mexico. Thread education is fascinating. I now understand the price difference in quality thread from okay thread. Everyone has their favorite thread and some think their machine sews best with one type or brand of thread.