Heater output? No. But it warms the truck up faster, and keeps it warmer since little to no air is being passed through the radiator. My truck has a hard time keeping a warm operating temp when on the highway, when it's below 40 degrees. The muff helps it out.
But be very cautious about when you have it on and your temp guage. Make sure you have an accurate temp guage.
I also have the raised vent for the heater air intake. I wouldn't say it adds any real air flow. But, it looks cool