There's a guild on my server who killed Garrosh on normal recently using 2 players from a different server. I thought it was flex at first but WoWProgress has them at 14/14 cleared now. They didn't transfer to and back again because the log shows their server name beside their own
Another odd part is the 2 from off server didn't get credit for the kill nor did they get the Ahead of the Curve achievement, while the rest did