So the competitive Dominion scene will be unfairly skewed towards playtesters for however long the adjustment period is, no?
It's totally true but mostly irrelevant. If you play a bunch right when the set comes out, you'll be most of the way there. The playtesters know what some serious players thought of the cards, but soon you will too.
I think it does matter for the league's A division, where Mic especially will have an edge. Outside of the league, Mic was beating you anyway, so the advantage is not so meaningful. Stef also playtested but not as much as Mic, and again was beating you anyway. If there's a tournament then jsh357 will have an edge too; he hasn't been in the league. I am not trying to put down the other playtesters; jsh357 just played a ton and might participate in a tournament.
I would personally try to schedule competitive stuff for right when the set shows up online, which, who knows when that is. But you know, it's fun to specifically be trying to figure out the cards competitively. I don't know if people would prefer banning the playtesters (presumably all of them, not counting the people who just got to play the set for one day at one of those two things) or allowing them; I would lean towards allowing them given the size of the community (small).