yea they have stuff in them

According to the MK Dev Team, they got the Gold Koffins there to point some good stuff to unlock. I don't think they're that good, but oh well...

Yeah, MR Boon himself stated that they are somewhere to start to get some decent things. And just to let you know the hints are not some kind of secret code they just point you out to other koffins to open. example XG = EX God, this lets you know that you unlock raiden by opening koffin XG. DUMB.