We typically go for 9-10 days and do MGM at least 2 times if not more. I think the issue is not having only 6 days to go but what you do in those 6 days.
If I'm not mistaken, you have TL & BB and Universal in those 6 days as well, correct? So, technically, you're only spending about 4 days for 4 parks. That's definitely not enough to get the full park experience unless you go commando on those 4 days.
You also need to account for travel time and wait times when planning your days. You may not realize it but if you go on Disney busses, it can take some time going from place to place. Wait times for rides can carve up a big chunk of your day too.
Take for instance RnR and ToT. You could waste 1/2 hour easy on just 2 rides. Then walking time to get to Star Tours, GMR, etc. I could never do all of MGM in only a few hours. MGM has a lot of shows too that if you don't time it right could eat up a few hours at least.