Does the resolution of your monitor effect the resolution of your headset? For that matter would one get more FPS if they unplugged their monitor from the video card after they launch steam? It just seems that the video card is using power to output to the monitor???
It probably will reduce your fps, but probably not noticeably.
You wont notice any difference. Monitors are not consuming anything itself and will only mirror vr headset picture.
If your monitor is connected with a dvi cable then it will be using 1 channel from your graphics card, where as a display port does not use that channel. The fps drop is minimal as others have said and it may impact it if you increase the monitor resolution too high, but if it’s low you should be fine.
Both my 4K monitor and my Vive Pro are connected via DP on my 1080ti. If and when I pick up a Pimax 5k it will be connected the same
Then you should be fine… Have fun when it arrives