Yes and NO.
The Laptop you have is capable of upgrading further but due to its messures you should actually stay like that. Also, Cards are not better for its Gbs of VRAM, cards are better for its clock speeds. And GTX 960 is one of the best cards you can ever had on that laptop considering that further models are EXTREMLY expensive. Like 3 times the normal price of a PC part of the same exact model.
As i said on top, 2 gbs of Video Memory is more than enough to ANY 1080p Games. My titan X SLI dont even use its 16 gb of VRAM, they only use 2 or 4 when playing at 4k resolutions...