I don't think it contributes much to sound if you just swap out the Y-pipe. The only reason I used the FX Y-pipe is due to me badly stripping one of the bolts
When I was looking at the diameter of the pipes between M and FX, FX was a lot more beefier. Plus, on the FX Y-pipe, the single pipe where the two pipes join is shorter than the M.
I took some shots during the day:
