No you really cant upgrade past the 4940MX. A 1080 SLI won't work since the 330w PSU wont handle it but dual 980m's or maybe dual 1060's as it specifies on techpowerup that its 80w max for one so 160 plus the CPU wouldn't hurt. I would also recommend playing at higher resolutions so the 4 Core's wont bottleneck the framerate. I also heard that they've released 16 series cards as MXM but dont remember if they finally released.
Hey I know this was a while ago but if you could help me I would really appreciate it I’m having the same issues, my computer keeps going into hibernate every time the graphics card carries out major work like games videos or other demanding tasks i have a 2015 ALIENWARE 18 4940MX  GTX 980M x2 SLI I want to know is it better to upgrade GPU to dual GTX 1080 SLI or a single RTX 2080? And will I need to do anything to the fans or heatsink. also can I upgrade the CPU to anything better than what I have 4940MX I know it was the top of the range at the time but is it possible to upgrade to something newer will the socket let me?  
18 R1, need advice for upgrading, #2