easiest way to find out is to call the service manager. I talked to all the ford dealers in a 60 mile radius.
My service manager told me that they have had problems with modded cars (chips in turbo diesels, underdrive pulleys, nitrous, etc).
For instance, flowmaster mufflers would void the coverage on the mufflers. A chip would void the warranty on the computer and the engine.
If the service manager says, 'if you change out x pipe and somethings happens to your engine down the road, your warranty will not be covered.' then you have your answer.
I think they can void your engine/drivetrain warranty with a catted x.
You've got to understand all the good and bad things that can happen with each mod and then be prepared either way.
I'll tell you what one service manager told me about gears. (this is what he said)
"Steeper gears increase rev capabilities, putting more stress on your .... Eventually, your're going to discover/find the weak link in your car. If that weak link fails it is not because of factory defect, it will be because you installed lower gears that put more stress on that weak link."
Of course, you've got acts/laws that allegedly protect you. But I would get the information from the service manager and then make decision accordingly.