Gtx 860m power in one scene

Framerate is not that simple. There are many factors that come to play, for example, the framerate can drop because the CPU is bottle necked, even though the GPU has no problems rendering the scene.

I guess if you want to do a simple, but crude and not very accurate test, you can continuously clone objects and watch the FPS, stop when the FPS starts dropping and you have your answer. But I would say this is a poor test, because each computer is different, and performance drops can be caused by different factors.