Here is a good breakdown posted by MrYellah a few weeks back when discussing Mat vs GMB.
Now to muddy the waters further, 2.4 is going to allow GMB/Mat bows to be used on A1 mercs but now they are limited in reaching those breakpoints since they will only have chest/helm to help them get there (and any sockets in those).
If IAS never matter on A1 mercs, then you would take the slowest bow which gave the biggest damage and I’m pretty sure that isn’t want everyone does.