And if you often shoot with the vertical grip, adding it to the F6 will add 35.495 oz [source: KR].
So this means that the F6 + MB-40 will be:
so a half a lb lighter for the F6, but it will be taller than the F5 with the built in grip.
To each their own. If I had a F6, I'm sure I wouldn't mind, but the price, for me, makes it not possible.