I don't know the exact number they switched on, but yes, they replaced the A-arms on the first few hundred that were already delivered with the cast arms (that were prone to cracking). They knew they could fix the impurity problem that was causing the cracking, but Ford also knew it would take several months, and they didn't want to halt GT production and delivery in the meantime.
So Ford bit the bullet (billet?) and decided to simply machine the A-arms from solid billet aluminum until the original aluminum casting process was fully updated. "Yeah, but that sounds expensive." It was. I've heard it added approximately $10K to the cost of each car with those arms. And you were thinking the $10,000 price bump toward the end of 2005 production was a coincidence, right?
Those A-arms are works of art. EXPENSIVE works of art. Sad to think of all the crumpled 2005s with those on them. Even if they've been fixed, and even if they were fixed properly, they didn't get no billet A-arms as replacement suspension pieces, I can assure you.
In fact, there's one way to know if a car has been wrecked. If it's a 1,700 VIN or earlier 2005 and it DOESN'T have the billet A-arms, something be up...