The Bazzaz could increase your fuel mileage, lower running temps, and deliver power in the areas you determine that it's needed. Those of course, are dependant on how much of the other aspect you are willing to give up. If you tune for mileage, you lose some power. If you tune for a cooler bike, you lose mileage and power, because you will be fueling more across the RPM spectrum, which may not be efficient for power. The Bazzaz allows you to adjust the fuel and ignition to meet basic requirements, but only to a certain degree. It does however, run circles around a power commander. The Bazzaz will let you specify fuel economy at XX Range, but deliver power above said range. It's pretty flexible.
Being that the Bazzaz is behind on development and production for the Z650, I am now looking toward Rapid Bike, which does the same thing, but to a higher degree. I will be able to adjust the timing and have finite control to the ignition and fueling well above what Bazzaz or Power Commander can do. It costs a bit more, but I'm impatient, and want it now.
If you don't plan on adding exhaust, or changing the air flow on the bike, you don't need to touch a thing. Even if you do buy a better exhaust, you may not need any tuning if it's a quality setup.
I'll have dyno results this Wednesday on how much the Yoshimura changes the air to fuel ratio on our bikes, so keep an eye out. We will know more about how the Z can handle aftermarket exhausts then, and what may be needed.