I have checked the fuel economy many times, 26-27 @ steady 65 mph on the highway. However, I think it's probably due to more than simply restriction free exhaust. I'm running MSD ignition with 8mm wires, ram air induction to gather plenty of cold air from outside the hood, we've dropped the engine temp to 130-140 degrees with hotter platinum 4 plugs to allow for 4 degrees of timing advance, K & N hi flow air filter, and mods were made via the computer to affect timing and mixture curves. The transmissions shift points were modified as well to balance power and fuel economy, shift points are now (during normal acceleration) first to second @ 25, 2nd to 3rd @ 40, and 3rd to overdrive @ 51, the trans does not shift early if you are going slow, as during city driving in heavy traffic, or when driving offroad in uneven terrain, when you let off the gas and it is in one of the lower gears, the torque converter freewheels so the rpm's drop and you coast, and locks back up and resumes the same gear and rpm when you push on the gas again. Basically, it's not just a matter of running straight pipe or getting rid of the thermostat when dealing with the more modern electronically controlled engines, it's making modifications that work together to acheive more power and better mileage, i.e. exhaust, cooling, ignition, transmission. When you radically alter one, you have to make changes to the rest to really achieve significant improvements. Interestingly enough, I have noticed a sideline benefit of running the engine extremely cool while using hotter plugs and improved airflow to increase the combustion temps in the cylinders... Little or no oil related deposit buildup in the engine, as running this cool does not break down oil additives if changed regularly, and very little carbon and related combustion chamber deposits. Spark plugs and piston tops, valves etc, are just covered in a light grey ash, little or no black carbon. I have pulled apart engines with well over 200,000 miles on them, and friends who looked at the heads and pistons couldn't believe they had more than 30,000 on them because they were so clean. This probably also contributes in part to the extremely long service life I get from most of my vehicles, even when driven and revved hard.
Dave