I think he is wondering if it looks disorted or pixelated since the iMacs 27 has a higher resolution than 1080p.
I think its looks good but not as a good as 21.5". Thats just a guess though
Don't worry, it will look fantastic. The iMac screen is awesome!
1080p is the quality I look for when I download movies. IMO the 27'' in general scales very well down to 1080p resolution, including games.