Well, I was about to cut out the ply for my centreboard case this morning, but I think I'll leave it till you have figured out what is going on with the balance with the stiffer yard.
Regarding fiddling with the ties along the yard:
Are you both thinking in the same direction? If I read Mik correctly, he's talking about letting the sail edge curve go back closer to what it was on the wooden yard by keeping the middle ties tight and the end ones slacker, so that the upper part of the sail has less shape, and is more like it was before the change of yards.
Perhaps Mik can clarify.
Ian