Fair enough, ill re check anyways. I usually do it myself. I fill up, use the car for a week or so, fill up again and divide the gallons it took with the km I drove.If there is any change to your mpg from this mod, i would put it at well under 5%, in either direction. The reason they are saying they dont believe your mpg claims is because 30-40% improve is huge. That's like saying you went from 30mpg to 39-42mpg. The part you removed isnt going change much of anything, very minimal restriction.
Just enjoy the new sound, dont worry about the mpg 
But if i use the computers avg l/100km, it went down from 20-22 to 14-16.
But again, ill re check. The point is my focus sounds sexaaaay hahaha!