I can't see how the mass backing will make the carpet fit any better. From how I understand it, it is just an added sound deadener and heat shield. I guess you could compare it to that Gator skin or Lizard Skin stuff that you put down before you install the carpeting. Personally, I've always used the factory type stuff without that added mass backing as my customers never specifically asked for it. I can't see how something that is added on would make it fit any better than what the carpet should without that stuff. Its kind of like saying that your paint will stick to the body better with a coat of wax versus no coat of wax. You can put all the add on stuff you want after the fact but if you don't have the right prep, that paint won't adhere to the body no matter what kind of wax or add on you put on after the fact on top of the paint.