I just found out by accident that you can get Realm First Achievements after transferring onto a server where noone got that Achievement yet without having to clear the instances.
So for Example:
You already got the 25 Naxx Clear Achievement but you were not the fastest one on your server.
You can now transfer to a Server where noone got that Achievement yet.
If you now enter Naxx 25 you should be getting the realm first achivement + title without having to clear the whole instance.
Should work for all instances.