The reason the M1911A1 had an arched MSH, while the M1911 had a flat MSH, was because coming out of WWI, the Army had lots of complaints about the 1911 shooting low. The arched housing was to fix that.
I started out with #1 on my XDm45 and was shooting low, so I switched to #2 and the problem went away. I prefer the feel of #1, so I switched back to see if having more trigger time with this longer pull had solved the problem. Unfortunately, I have not had a chance to test #1 again. I am suspecting #2 will go back on. Report coming....
To this day I run arched MSHs on all my 1911s. First thing I change out when I get a new one.
Meanwhile, if you are shooting low, changing to a larger backstrap may help.
CX