The world of logs thing is easy, just take a look at a page like Malk normal mode (
http://worldoflogs.com/zones/Siege_o...mmar/Malkorok/ ) and you see the median rdps is about 1.4 mil. Assuming 5.5 dps, 2 tanks (each about 0.75 of a dps), and 2.5 healers as the average comp, we can divide that by 7 total effective dps = 204k dps, while the top logs of 10m normal is 485k ~= 42% for this one boss specifically. To be fair, the average player in LFR or Flex hasn't even killed normal Malk yet so this is likely to be skewed to be in favor of the average players' skill, so I dinged it a few percentage points. Healers also tend to do some dps, so again, a few percentage points dinged. Probably a better estimate would be looking for data on multiple classes on wow-heroes or something that provides a 50% median number and compare it to the top ranks for each fight, then get a median on the resulting percentage averages, but I'm too lazy and 35-40%'s close enough.
As for endless waves, there's a wide variance among the classes in how many waves they can do as you can see:
http://www.wowprogress.com/char_stat...ield.pg_damage , but the worst performing classes seem to top out at around 50 waves of endless (druid, paladin, shaman). Using those as a limit, 30/50 is roughly 70%.