MB-D12. Go for it, it improves the ergonomic greatly. The Batteries. This decision I took last week and I know that I will never regret it.
The EN-EL15 is Mickey Mouse stuff. Granted. The proposed EN-EL18 is a far better choice until you look at the price: $220,00. But things are getting worse when you have a look at the price of the charger: $458,96. Well, all is not too bad; you may buy the compartment cover BL-5 for $33,50.
Guess what, I am using Eneloop batteries in all my SB-900's and I started using them in my new 800E. Good, very good decision! You may buy an other EN-EL15, but then, you will have 2 Mickey "Mice" onboard!
The whole problem here is CAPACITY! The EN-EL15 is too weak. And getting a second one will not make you twice weaker but twice unsatisfied. 1. No greater capacity and 2. poorer. For better power to cost ratio, go Eneloop.
I don't mind paying where it is necessary but I feel that sometimes some are pulling too hard on the rope…