If you already have the optional 3:55 gears, going to 3:73 won't be worth the expense. What do you have now?
Lowering springs can require alot of other parts and I suggest buying a system from one company such as Steeda or Roush that has tested them together.
Aluminum driveshafts are notorious for vibrations, but this one ,
JDM Engineering has had the least problems from my research. Sifter I would use MGW.
That being said, it depends on what you want the most as to which comes first. Speed, do the d.s and gears first, (although a good tune would give you more power than both of them). Looks and handling, do the springs.