For me, the changes I made to the bike were to make it more powerful and efficient. The exhaust cans were a no brainer, far less weight and a bit better breathing. I put on the S&S cans with the db killers in so they are not so annoyingly loud. I prefer them because they are a factory option and are muted enough not to get me pulled over in NY State and certain municipalities here in NJ where they have strict noise laws.
Just as an FYI, I did try less expensive cans, and they made the bike run worse than the stock cans with the db killers, and significantly worse without the db killers. But they made a lot more noise which gave me a headache on long rides. The S&S seemed to work better.
I already had the Booster Plug and the DNA filter installed with the snorkel deleted to open up the intake. The full combo made only a very slight difference in performance, but did make throttle response more crisp.
Then there's more to do if you want a bit more power. You can get open head pipes to increase exhaust flow, and also save a ton of weight. Hitchcocks makes a nice pair. At this point, you'll need either a PowerTronic or Power Commander to deal with the higher air flow through the engine.
And why stop there. Hitchcocks also make a very nice cam that will beef up midrange torque and high end HP.
But wait, there's more! At this point a nice set of high compression 650 pistons will certainly make everything better. And I'm here to tell you that yes, they can and do make a very noticeable difference. Even without dyno tuning and just a canned map in the PC had it making 57 hp instead of the 40-42 it makes stock at the rear wheel. The character of the bike is still intact, with even more low to midrange grunt, and much higher HP numbers, and still returning 60+ mpg even while riding with great enthusiasm. It doesn't make more heat, that I can feel, over the stock motor, and not so much power as to be overwhelm the brakes, frame and running gear.
It's all a slippery slope. Exhaust cans are the entry drug. But tons of fun along the way, and the bike lost about 30+ pounds. Now if only I could do the same on my body.