Just passing on something I stumbled over when I noticed a discrepancy in my Waltz #s. If this was already found, my apologies if I just missed the memo, but it's always handy to know caps on +% boons. /whm my base waltz3 is 397 and was using Phurba, Sonia's Plectrum, Roundel Earring and Dancer's Casaque each time.
Unweak on my way to campaign NPC and use Waltz3 before tags for 500 (+26% in gear). Waltz3 again after tags (+36% from Plectrum campaign bonus), only get 516. Full +10% increase should've been ~539. Take some damage and reuse Waltz3 w/o Roundel equipped (+31%) and get 516 again.
Crunch numbers and get:
516 / 397 = 1.299748 (and in reverse 397 * 1.30 = 516.1)
So I'm pretty sure potency can only be increased by 30% from gear enhancements (no clue on other things that can boost it though).
Considering the most +% available outside of a campaign battle is +31% (Phurba, Roundel, AF1 body, AF2 head, Plectrum), it isn't THAT bad. It will definitely make easier judging of any new Potency+% gear SE chooses to release.