I wouldn't ever OC the CPU on this machine as it al runs very hot. I would suggest an undervolting for the CPU to keep it cooler with no performance loss. As for the GPU, you could use NVidia Inspector and make an OC profile with +120Mhz core clock and +500Mhz (total 5500Mhz) and just activate it if you feel the need for more fps in a certain game. I didn't find any game that required any OC until now, and I'm also running an underclocked CPU with only 3000Mhz when all 4 cores are running. I like keeping the temps as low as I can with little to none performance impact.

How muc unservolt is recommended? And is it aafe to pc the gpu? Mind posting a small guide on the cpu undervolt?

Undervolt is safe, but OVERVOLT is not safe. If OVERvolt you can hurt your cpu, but with UNDERVOLT you can't!

Only thing that can happen when you undervolt is that you get black screen and pc restarts if you have undervolted too much.

I am using MAX undervolt on my 4860HQ, -90/-100 mV, and had no problems, you maybe can try -80 mV on both cache and dynamic cpu, and see if you get a black screen after 2-3 days of using your laptop.

Sometimes play a game, sometimes leave it idle like you are going to take a shower or something, come back, if it is still on, great 🙂
You have reduced your temps by 4-5 degrees, no performance drops !

Just use intel xtu, and drop the sliders on -80 mV on both dynamic cpu voltage and processor cache.

I have maximum -90/-100 mV and no problems. Try, if you get black screen, bump up by +5 mV and try again.

You can't burn your processor with undervolt, remember that !

Here you are, a tutorial for the CPU Undervolt: http://rog.asus.com/forum/showthread.php?52467-Got-your-new-G751JY-G751JT-G751JM-Post-your-review-te...
If you can go to -90mv/-100mv as I did in the tutorial then I recommend keeping more safe values like -80mv and -80mv because I did eventually have problems with -90mv/-100mv and the laptop has restarted a few times when I was playing Crysis 3.

As for the GPU recommendation, I would suggest no OC at the moment since the GTX980m is already powerful enough and can handle all current titles at very high details (just don't go too high with the AntiAliasing. 2xMSAA would be more than enough).
But, if you do want to OC the GPU later, then I would recommend a +135Mhz to core and a 5700Mhz memory. But, you have to check if these values are safe for your laptop as they are for mine, because the chips can vary from user to user. The GPU OC won't do much damage as the GPU temp won't raise too much, but do not go as far as changing the Voltage and do not exceed with the Memory OC. Unfortunately, I do not have any tutorial for the GPU overclock but I am usually using Nvidia Inspector so I can easily make OC Profiles that I activate only when I need them. You can find many tutorials about Nvidia Inspector on the internet. Since you already know the values that I am using, it will be easy for you to make an OC yourself.

Keep in mind that Overclocking could void your warranty if the GPU is damaged in the process. The damage usually appears from too high temperature, so you should keep an eye on the temperature everytime you are testing an OC.
The Undervolting is quite the opposite. Undervolting is safe and it will not void your warranty as the process can't actually damage the chip, and the guys from the service can't even verify if the voltage was changed with a negative offset.