I feel like the biggest accuracy improvement comes after you get all the oil blown out, and the rifling gets a decent amount of lead transfered to it.
Obviously the POWER increases some after the piston seal seats, but I doubt that effects accuracy much. I have put those blue Australian piston seals on both of my break barrel guns with a very thin coat of Moly, and it gave both of them a 30-40fps boost immediately, that hasn't changed one bit in over 1000+ shots.
Those piston seals are the bomb! The standard "doughnut type" seals are junk IMO. The only thing they do, and I think it's the reason why the manufacturers still use them, is they leak enough that it doesn't self destruct if you dry fire it. But the blue solid seals definitely make the gun pack a punch.