you can go to gearingcommander.com to figure your actual loss. If I had to guess you would lose around 10mph off your top speed. As for your speedo you can go here to figure out how much it's off and use the link in the product description to take you to the calculator.
Stock gearing on an '08 is 17-45 , not 17-47. Going to 16-47 will reduce your top speed to roughly 165 from the stock 183. Mileage won't be too bad,but you will be shifting alot more. I prefer stock gearing, if not taller. (more top speed) 165 is too slow for a 1000 IMO.
With gearing on a 1000cc bike you can't really gain anything without losing something somewhere else. You're not really gaining anything, but just moving things around. The key though is to set up the gearing to what suits your riding style. If you like top speed more then anything then stick with stock. If 95% of the time you're under 150mph then most people would prefer more bottom end at the loss of say 10mph of their top end.
If you think -1 in the front is too much then you could do +2 in the rear which is about 2/3rd's the difference of doing a -1. Doing just a +1 in the rear you will barely even notice the difference over stock. In fact I would wager that if you take a stock bike and one with +1 that you can't even tell the difference. +2 in the rear is the smallest change I would recommend to where it's noticeable.
agree with others about dyno, don't need it. disagree with top speed problem. i'm -1/+1 and still top out at about 170mph. that's plenty, not sure bike has enough power to get much higher, and if it does, there's not enough road here to do it anyway. i can tell u that the increase in acceleration will help you forget that top speed loss. unless your a top speed only person. on the track, on the road, anywhere you'll get much more joy and benefit from gearing. bike will be significantly quicker.
as for mileage, i'm guessing it's the same as a car. less work but more revs, slight negative outcome, but not as bad as people think. i've never gotten more than 30mpg outta my bike, before or after gearing. that's just cuz i of how i drive, not the gearing i have. if it's that important, sell the bike and put that into a gas account.
try going as fast as it'll go, with the stock gearing. see how high u can get it and what number is comfortable with u as far as being attainable.