I am someone who 5 boxed the heroic end time dungeons at the end of Cataclysm. I recognized almost immediately when stepping into WoD dungeons that they would be different and that has turned me off to 5 boxing them.

I've never had an interest in going at something beneath me like old dungeons and raids that I massively outgear. For this reason I switched to arena. We can never stop learning there and it isn't as annoying as PvE can be.

The one dungeon that convinced me to not bother I think was Skyreach. The stupid floor maze on the way to the second boss. Things like that I just don't enjoy. It is laughably easy on a single character and superbly difficult on multiple characters. One of those design decisions that make you scratch your head I guess.

Anyway, if you are really interested in sticking out multiboxing and like the complexities I would try arena. I like 2s for tight control and broader key mapping. I find 3s are a bit tougher for one person to parse whats going on all the time. Not impossible though. My turbo MW group had some success two seasons ago.