If you install "ANY" aftermarket product to your 08 engine (exhaust, chip, air intake etc..),
warrenty will be null and void. It's that simple.
However, I was told by a Ford service manager, if you install any aftermarket parts, exhaust, intake etc.. keep your old parts, then if you have engine failure, re-install the factory parts (all of them), throw a little dirt on the items you replaced then there is a better chance there will be no questions asked.
As for the extended warrenty, dam if you do, dam if you don't.