Do you need your merc to deal dmg?
If you answered yes, then upgrading all your other items on your char should be a priority over investing fgs into a better infinity.
If you are effectively min/maxed and you want to also min/max your merc, then the difference in dps is so minimal in the end that you should prioritize the leech he gets from different bases.
Leech depends on flat dmg, not CB dmg.
And keep in mind that this is a fast paced game, moments where you let your merc stand there and jab multiple times in a row a very rare or inexistant. So it's not about dps, but about how much dmg he can deal with 1 jab, while the end of its attack animation is mostly gonna end up during your next tele, so it doesn't really matter if he attacked faster or not.
Assuming CoH + either Cure of FF (0ias setups)
GT will perform first jab attack at frame 5, 2nd one at frame 9
GPA, with higher base dmg, will perform those 2 attacks at the same frame
only the end of the animation lasts 1 frame longer, and this will get ignored 99% of the time
And mancatcher would perform the 2nd attack 1 frame sooner, at frame 8, but with way less base dmg, and most situations will allow the merc to perform the same amount of attacks (2 or even just 1) in the same time frame anyway.
So GPA is the best base.
Holy freeze and decrepify say hello and this is where you need that merc in most cases to be fast and deal damage.