Should have no effect on mileage if you ride the same way as before. Problem is, since the bike runs so much more smooth and responsive, you will probably have a tendancy to be a little more liberal with the throttle than before. This will equate to lower fuel economy, but a bigger &#$( eating grin on your face, Which is worth more?
I have SISF jet mod too, and as stated before, if you can afford it, do it!