Lets go do a dyno between our bikes and findout, or maybe I will go do one just to prove you are not 100% accurate?
With the ONLY change being the muffler? If there was more than a 0.5% difference in HP, I would be surprised. And tests would have to be on the SAME bike, because I estimate the difference (if any) to be below the range of statistical significance between bikes or even between dyno runs.
There are good, better and best when it comes to slip ons and systems, and no not all are loud screaming race exhausts. The stock unit is in fact hideous, Kawasaki needs to do much better, lighter "and smaller" for a bike this nice.
Oh, I know not all are screaming, I am just being contrary. There is no question it is big; with the bags on I think it looks fine to me, and I will never have the bags off. Weight- well, it is not light, but overall I don't think it matters that much. For example, I weigh only 150 lbs myself, so my bike is already at least 25 pounds lighter than average riding condition (perhaps that is even too low an estimate)
I know every bit adds up, but most of us are not on the track where 10 or 12 pounds might actually matter. I will take the 0.001 second penalty.
Now, if I could lose 10 pounds of fat on my stomach and sides (and keep it off)- THAT would be worth many times the price of a muffler!!!