[amlebede]

It is certainly possible to lower an XI. It has some advantages like improved traction on good roads. However, it will be a disadvantage in bad conditions (ice, snow) and may even cause damage to your car in the worst case scenario.
Improving lights is always a good idea. Putting in a Xenon conversion kit if you do not already have one is an advantage.
Put in a spoiler if it will not be purely decorative but will actually serve a purpose. I personally wouldn't put custom rims unless they are sports wheels improving performance but it is your choice.