There is two ways that I can interpret your statement. First is that you get FPS drops that drops the frame per second as low as 20-30. In this case, I would say it's a common problem, because no matter how good the driver is, Any card will have drops. Placing the game file on an SSD can be pretty helpful, but will definitely not going to fix the issue entirely. The second case, being the 20-30 frame per second as average, I think this is pretty much underperforming, Do you really think it is underperforming?
My Logical guess would be it's a driver conflict. Try a clean install to the latest driver and see if it may help. But if that didn't help then my guess would be the PSU not delivering enough power. Although it may be unlikely and a bit illogical, It'll be nice if you tell me what your PSU is. If both didn't help, then it's a defective card.
Do you have a tool which can monitor GPU activity? If not get one and run it in the background while you play sleeping dogs, after some time you have experienced the bad frame rates get back to the tool and see whether the card is running at 100% and whether the GPU memory usage is at 100%.
If the GPU usage is less than 80-90% then something else is responsible for the slow down.
If the GPU memory usage is almost 95-100% then the memory is less.
Otherwise there is a possibility that the GPU has gone bad, and you may RMA it, if ASUS accepts it under such conditions in your region.
Do try the tests I recommended aside from the temps. Temps can be mitigated with better airflow or custom fan profiles, if those don't cut it and you don't have a higher ambient temperature then its the GPU which is at fault.