Not sure about how many shots with the DSTE battery as I have never run out on a day shoot. But it would certainly be in excess of 500 on my D810.
I have not done a longer shoot with that battery in as I tend to only use for it Day long shoots.
I will be buying another one now I know it fits into the Mieke Grip. ( Need to be carful fitting it though as it is a tight fit. ) I have to buy another anyway as I have somehow lost the charger for the Battery :-( Only have the car charger left and that is not that useful. The batteries come with a charger. ( US plug though, but that is not a real issue )
They will get a lot of use later next year or possibly 2018 as we are making plans for a 4/5 week trip back to Africa and charging batteries can be a challenge in the bush.
Also I checked T O Shooters info about the customisable extra button and he is correct. I can customise that button via the MD-17 button options in the camera settings.