Ford has basically not installed 2.5" pipes on anything but bigger trucks and top line Mustangs. I guarantee you that Ford didn't start any time recently by sticking big pipes on a mid sized SUV.
2.5" pipes are just about an ideal size for a stock 4.0 or 4.6 engine, front to back, duals and tail pipes. That's what they should be for best efficiency. Now do you want to argue the inadequacies of a stock exhaust and duplicate it, or shall will make it better?