I have heard that they're noisy too, but I honestly can't comment on the piston noise thing. I swapped from my stock y7 head to a z6 head with a zex cam and isky valvetrain the same time the rods and pistons went in. Between the rattling of my valvetrain, and the open downpipe I can't really narrow down all the noises going on in there, but there isn't any decrease when it's warm if that makes any difference to you. My engine had 70k miles on it when this work was done, so maybe some of the higher mileage cars that keep stock bore have a more noticeable racket due to poor clearances.

Most guys seem to have no problem with the 75.5mm setup. Good luck with your build.