No kidding... Swapped it out with the one piece Motorcraft ones and surprisingly the car is running like a raped ape. Feels like it has gained 50hp back. One of the plugs did break when they were trying to remove and it took them 6 hours of work to finish (didn't charge extra though TG).
Ford has a nice solid history of screwing up Mustangs. I have a 2002 GT which I love dearly. BUT, changing plugs is a serious challenge. Seems Ford in it's infinite wisdom cheaped out on the heads. You have a total of THREE WHOLE threads in ALUMINUM holding the plugs in. And... Oh yeah, the plugs are also two piece. Not to mention they aren't in the usual position on the heads alongside the exhaust manifold, no. The 4.6 has them up on top just a tad under the fuel rails, and pointing...