My guild went through significant efforts to transition from 10-20 man. We poached players from 10 man guilds that broke apart, we negotiated guild mergers with other 10 man guilds. The result was... not great. For a tightly knit 10 man guild to suddenly double their roster size, well it's not impossible. We did it. But at a cost. Trying to double the roster meant we couldn't be too picky about our recruits. As a result, raid performance dropped, we went from a 10 man guild clearing Mythic (or Heroic as it was in MoP) content to a 20 man guild that maybe cleared 70% of a raid on mythic before the next tier came out. A lot of the people from our original group got fustrated with this and stopped playing. We are a guild of folks who would rather play together than with anyone else. So when faced with the choice between joining another guild or quitting the game, people always chose to quit.
So for me, 20 man Mythic was a failure, i didn't see how having 20 man made things easier to design.
Going into legion, the guild leader ship is considering stepping down from Mythic for a time. Best proposal so far (imo) is to form solid 5 man groups for the new CM Dungeon format with Heroic flex on the side and hopefully developing those groups until we can make a solid mythic team out of them. Gonna take time though!
So in short. We made a HUGE effort to transition to 20 man Mythic, more than most guilds I think. It wasn't a success.