If further nerfed it might be a little useless with a single copy. It's on the OU side of the game nowadays, but if anything it might need a bit of a cost increase.
Honestly, despite the fact that I suggested the 25%->20% change, I wound up voting for leaving the 25% where it was and just giving it a cumulative cap, since, like you said, that'd make one of them less than useful and the problem is mostly when they have 4 of them in play.
Increasing cost would also be alright. I don't think a
huge nerf is needed at all, just a relatively minor one.
Something else I just thought of: changing it so that when the boost triggers, rather than just ignoring
all shields, change it to spell damage so that it'd be reflectable. Or, cap the evade at 50% and leave the damage/shield uncapped.