It probably is a few hundred potency better. I think I ran some numbers on it a while back, which showed like 600 Potency if all the stars aligned. But I've started so many fights where you wipe like 2 minutes in and then you just blow all those timers out the window. That's a lot of work for like 5 DPS. (8 minute fight)
I guess SMN could have a few different options. Like:
Standard Opener Garuda
Standard Opener Ifrit
Max Opener -> Insert Gil here.
Although OP did ask for the best possible SMN opener, which I guess is subjective lol. Best for me is, costs no Gil. (RS onwy) But, your right, the best possible would include, XPOT, RS, BVFoes, Trick Attack, Poison Pot, Contagion, SC Ifrit - Rouse/Spur Enkindle.