Well, the DAJ isn't necessarily a power booster in and of itself. What it does is act like a jetting device without having to change jets. I had it in my 04 650V2 and loved it. You should only really install it if you are running a tad lean or more (such as due from snorkeling or the like). I increased my air intake to 2" (thus leaning it out) and installed the DAJ and it worked great. I live in MN where the temps go from 90+ to 0 in a year, so the oxygen levels in the air change drastically. That's where having the DAJ really helps as you can change it as needed for optimal combustion.
So, in short, if you want to boost your power, open your air intake to and install the DAJ and it will increase power (more air+more fuel=more power). However, if you don't increase the air intake, you're going to run rich.
Hope that helps!