Since the increase in HP comes via boost from the turbos your right foot controls your MPG. Only should you access the additional boost(power) will your MPG decline. With that said, if you are like me you will inevitably make use of the extra power. As a point of reference with my N54 (335i twin turbo), I averaged 22mpg under spirited driving in stock form. In stage III FBO trim, making 40% more horsepower, mpg was 17 on average. This is about a 20% decrease. Now, with only 10% increase in power on the N63 I'd put mpg decrease at around 5% - 10% with your right foot being the largest variable. This assumes that the efficiency curves of the engines are similar - which I suspect they are.