Why wouldnt you get better gas mileage? Your engine is working less to get the same amount of spent exhaust gasses out. Its now more effiecent, gets the same power with less work (or more with equal work however you want to see it), thus should consume less gas

. Ibe never seen a drop in economy only increases. If you where to increase the fuel flow, duty cycle, or change the gearing the economy will drop though...
But bolt ons basically only make your engine more efficient.