At idle, you should feel the exhaust pulse. It will come out constantly but you should be able to feel it pulsing. If it feels smooth with no pulsing, then you have a clogged cat. The reason is a working cat is not an obstruction so the exhaust flows through freely, but a clogged cat will not let exhaust pass. A clogged cat forces the car to pressurize the exhaust in front of the cat, then force it through, which means the pulses are ending at the cat. We're not talking huge pressure here, but it doesn't take much to remove the pulsing from the exhaust stream. An exhaust shop can also check and most will do it free of charge and you don't have to worry about them lying about it because if they get caught by the Feds changing out a working cat it's 25 grand or more in fines. They're legally only allowed to replace defective cats.
IMO you have multiple issues here. Assuming you have the stock size tires on, a taller tire will slow the speedo down for the speed you are going which will make it look like you're not going as far as you are. Lazy O2 sensors will knock as much as 3MPG, perhaps more, off your fuel economy by making the car run rich and won't necessarily set a code. A torque converter with a failed lockup will cause the car to lose mileage by not locking the engine and transmission together, but I'd think there would be a code set for this. You say the coils (COPs) were replaced under warranty, I'd consider them suspect and would replace them all with Motorcraft along with NGK platinum or better plugs, making sure to gap them correctly.
Finally, you idle the car to warm it up? How long does the car idle? When idling the car is getting 0MPG, and if the car is idling a long time this alone will kill your mileage. I never idle my car more than one minute on the coldest days, I just get in and drive. Instead I'm careful to drive it very, very easy until it gets warm. Until it's warm I don't go more than just off idle on acceleration and keep the RPMs at a minimum.
^)
