well, driving a big car always do have more disadvantages on using on city.
only will see your car save on petrol while traveling on highways or any road that do not start n stop.

as the petrol wise yr car it is too early to see or determine does yr car is consuming high or low in petrol usage.
You have to see the black oil that you changed out does it have any things on it? as see the engine have any metal dust on it.

If your car is lack of power then your car maybe it is time for servicing. No need to worry too much as your car is in warranty
You may tell them your problem on it as tell them to do some tunning.

Bumpy when going through yellow stripes , as maybe you dislike the feelings of gantung or springs. You better do not do anything with it while will void the warranty.

Body kit , as you can do some modification as the price i do not know very much sorry.

You can spend some money on Rims, Japanese rims.