I have a good LG, a cheap Goodmans a Sharp and another at work which I'm not sure about, all that I use regularly and others less regularly. Some are connected using 720p and some 1080p.
its not really possible to answer your 'does it fit 100%, pixel for pixel' question, because TV's don't work the way you think they might do. TV's do something called overscan, which results in the picture being displayed bigger than the screen, so a small portion at the edge of the picture is cropped away. All TV's do this to a greater or lesser extent. Whilst I am therefore unable to see the edge of my picture I can in all cases confirm that my mirrored iPad picture is cropped.
Because, this might hide something you don't want it to on an iPad screen, unlike a movie where it doesn't matter, Apple have added a setting to correct for overscan. This setting basically shrinks the picture by a set amount. Because each TV set overscans differently, the amount the picture appears to shrink and the size of the black border introduced by this varies from TV to TV. Apple have likely corrected based on the worst cases of overscan. I can confirm that when I turn this setting on when using my iPad that I get a different amount of border on each TV.
if you are able to adjust how the iPad looks on your TV using your TV settings, it's the TV that is complicated not the Apple TV, the Apple TV has 2 related settings only.
so far as I am aware the iPad sends a 720p picture to the Apple TV, but this doesn't really matter, since whatever resolution is sent to the Apple TV, the Apple TV scales it to the resolution it is set to output.