I am using the same model with 16 GB RAM.
I asked my engineer from authorized hp dealer and he said that i can attach an 128 GB SSD or 256 GB SSD, depends on my choice. Its totally support and will increase my opearting system working speed and i can also attach my 1 TB HDD with it.
As for the battery, i have the same issue also, but that's okay. I think the battery is ok and perfect and there is no problem with charging, i think its because of the heavy configuration of the system that the game lags, when we cut the power, system starts saving battery by cutting usage of memory on certain tasks, so may be at that point, nvidia graphic card is not getting that much power.
Well, I am loving this configuration and laptop working. Hope You find my answer useful.