Piped sleds make way more power and use way more fuel. I don't know what mileage I get with my single pipe 700 either. All I do know is that I get the same or better mileage than my friend on his EFI Indy 500. Last year as well, I rode with a F5 in off trail conditions. We were also within pennies.
On my '97 SX 700 with aaens i got 15-16mpg all day long, riding the way i ride On my '01 SX 700R with Simons triples i couldn't get any better than 11-12 or the plugs would start getting dangerous. Different pipes have more efficient scavenging than others. Stock pipe with a good clutch and jet setup can pull 14-16 no problem.