It dempends a bit on how much memory and CPUs you have in your server.
For singel CPU servers NUMA is not even an Issue. On two CPU CPU servers, its not that big of a deal either (on modern CPUs the interconnect between the CPUs is fast enough that the performance diffrence is not an issue). However if you use 4 physical CPUs ore more, then NUMA can be an issue.
What kind of server do you have and what performance issues are you having?
My view is even if it only gives you a 2% improvement on a 2 CPU box why wouldn't you do it? If a simple setting give you an improvement you are getting a better ROI on your investment on both the hardware and Qlik.
I would say that thoose 2% is within the margin of error. In a 1 or 2 CPU enviroment, NUMA have no pratical use and i would guess that it doesnt make an difference if its on or off. But as you have stated, there is no penelty of turning it off either. Just dont worry about it at all