Okay I've made some progress!
I found the culprit of all my GTX 980m Stuttering issues, it's the Gigabyte Smart Manager software!
if I uninstall this software, the laptop runs smooth like butter, no stuttering in SMITE at all, perfect gaming.
BUT - without this software I have no sound!? no sound at all, the sound drivers are installed, the realtek drivers are installed, but still, no sound.
This makes no sense to me, but at least I now know it's something to do with the Smart Manager that's causing these issues, so I've narrowed down where I have to search for a fix.
The task at hand now, is to get the laptop working well, -with- the Smart Manager software installed, (besides sound, it also controls the keyboard backlight, fan profile and some other core laptop functions).
Hopefully Gigabyte support can give me a hand? - we'll see.
Strange how these forums seem to be devoid of all life.. - alas I hope this posts appears in a google search!