If your car is like mine, and I assume it is since it is basically the same car, just one generation older, most of the noise will come from the tires.
Generally speaking, the wider and sportier a tire is, the noisier it will be.
A set of quiet LRR tires in the original size will do wonders for noise and fuel economy.
Also, I've noticed that removing the parcel shelf noticeably increases cabin noise. If you have it off, try reinstating it.
Sorry for the somewhat off-topic nature of this post, I'm just trying to think of things you can do without adding weight.