Air leakage is significant source of high energy costs and uncomfortable homes. Look at air leakage like a slightly open window, maybe open just enough that you could barely slide a piece of paper under the sash. Now image every window in every room of your home open that same amount. Maybe the front door as well. At a glance, it would look OK, the window is mostly shut, so how much could you really be leaking?
This is like putting on a nice warm, fluffy jacket and deciding not to zip it up. Yeah, you're mostly covered with insulation so it will feel warm, but without sealing up that zipper, a significant amount of heat will be lost through the opening. This is precisely why you unzip a jacket or sweatshirt when you start to get too hot.
So if you were to take 6" of insulation combined with an air sealed attic floor, you would have a far higher performance than 12" of insulation without air sealing measures. So in this situation, less would be more. The most important step in insulating is air sealing. This is the step where you zip the jacket or lock the window. This ensures air does not leak through tiny gaps. This keeps the outside air out and the inside air in.
Some people consider air sealing to be the cherry on top. But in actuality, air sealing is far more important. Air sealing is the ice cream cone to the sundae, long before the cherry. It's the zipper on the jacket. It's truly that important to the process. It's the foundation that makes or breaks a successful insulation project.
So, in closing you will never get the results you want if you don't do the correct things along the way. Insulatiing without air sealing is malpractice.