Here's a link to the 206H POH with G1000. It was the last non-turbo 206 made. It should be able to answer many questions, looks like you've got a 1417lb useful load. 92 gallon fuel capacity. Just a quick look at the 8000ft charts looks like you can pull a 139kts at 15.6gph at standard temp. Cruise speeds at various altitudes seems like you'd get 135-140kts no problem between about 14-17.5gph depending on the altitude. My 182T does better than book number speed at lower than book number fuel burn so I'd trust what the POH says.
http://www.redskyventures.org/doc/c...07_NavIII-G1000+GFC700-POH-PIM-fromCessna.pdf
I've never flown a 206 but I've been highly considering upgrading from my 182 to a T206H