One of the top progressed guilds in US is Midwinter.
Look here for their recent Mythic: Dragons of Nightmare achievement list:
http://us.battle.net/wow/en/guild/sa...Midwinter/news
Of the 20 raid members... 19 had legendaries.
Of those 19, at least FIVE had received at least 2 legendaries, 25% of their raid (check armory and news feed):
http://us.battle.net/wow/en/characte...romicidez/feed
http://us.battle.net/wow/en/characte...llennia/simple
http://us.battle.net/wow/en/characte...hebelle/simple
http://us.battle.net/wow/en/characte...hebelle/simple
http://us.battle.net/wow/en/characte...as/Soaksi/feed
So legendaries do have a HUGE impact on progression... and somehow this guild managed to stack near 100% of the raid with legendaries.
5 out of 20 have received more than one legendary?
Thank you Blizz... this had made this game very discouraging.