The missing 5th is significant but not really a big deal until you hit outland. The old world instances are all pretty much a pushover until you hit Blackrock Spire. You will have some issues with elementals, such as Maraudon. Princess is a huge PITA with a shammy caster group. Not such a big deal with all enhancement shams but still a problem.

As for whether it would be efficient XP for you, that all depends on what you find boring. For me, I'd rather quest than grind instances. For some, they prefer to grind instances instead of quest. All up to you. I'd say they'll both be pretty similar once you get your macros and all that set up properly.