After thinking about it more I can see how you might pick up some weight in the front section. Base/Sport/Touring models have a Y pipe not a dual there. The Nismo exhaust and most aftermarket that are full cat-back will have a X or H pipe which will weight a little more due to more piping.
I was surprised to see the weight of the stock at 54, I'm guessing thats due to the single Ypipe. I measured the NISMO at 66 LBS and the HKS at 60 LBS. It's a weight savings for me
