+1. Rearward of the O2 sensors/catalytic converters have no effect on warranty.
Even going forward of those doesn't void the warranty -- Ford could just refuse to fix whatever problem you have that it can reasonably blame on the aftermarket exhaust and the computer re-tune it required. Of course, blaming such modifications is always their first instinct . . . and probably makes good business sense from their perspective
But staying to the rear of the cats is generally accepted as okay by the dealers, as it can't affect anything else. You won't get any grief.