-its the header that brings the power not the can. The can just makes sound. Jetting with a stock pipe will help with power. Jetting with a slip on will add power from the jetting and add sound with the slip on. With a full system along with less restriction you'll lose 14lbs of weight or so as well which helps with the power to weight ratio. Every little bit counts.
-Now with money and basic mods you can get the ninja from 27hp up to say 30hp. Do you think adding a couple hp is worth it? Do you think it will be noticeable? Nothing done to our bikes will make a big difference. Noticeable yes but remember we're not starting off with much in the first place.
-If it was me and I was going to spend a couple hundred for only sound then I would spend a couple hundred more to get some added power and weight loss to complement that sound. And jetting would then be mandatory.
p.s. Yoshimura For The Win!!!