don't really need G-sync if your running 200-300 fps. I'm not sure why G-sync would drop your frame rates by over 100fps though. That seems kinda odd to me since its not really a performance killer. I would think maybe something is not setup correctly. I'm using the 279q and it get straight 165 hz when im on CS:GO(gtx 1080). It should just automatically lock you at 180 fps when g-sync is on due to the frame rate matching the monitor. Try changing your v-sync option in game on and off while g-sync is enabled and see if something changes.
I7-4790k OC 4.9 1.375v
MSI Gaming X GTX 1080 OC core:2055Mhz Mem:5504Mhz
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