If you take any movie/video that is not formatted the same as the iPad (4:3) and display it on the iPad, you will get black borders at the top and bottom for anything other than a 4:3 format (like the old TV format). Hence, you don't really want to force any movie to fill the iPad display. If you do, you will either chop off the left/right sides or stretch the image.
Just because the iPad 3 is designed to display HD content (1080p) doesn't mean it will look just like your HD TV. If anything, what you will see on the iPad will look dramatically more brilliant and crisp because it has a higher DPI than the TV. However, this doesn't mean there is more detail in the image. You can't get out more than what goes in.