I use the Nvidia card on my P50 with Linux (Mint) and the results are... OK. The biggest problem is that when it resumes from suspend, about 50% of the time it forgets that there are external monitors connected. Sometimes it forgets both (falling back to the laptop panel), sometimes it forgets one or the other. Sometimes it enables both, but forgets to apply the rotation, so the positioning is all weird. I have to re-enable and re-position both external monitors, and then move all my windows around to where they're supposed to be.

I think the answer is likely, "it depends." I too have a P50, and mostly leave it with the integrated Intel graphics. The Nvidia is needed for HDMI output though.

In any case, I do fire that up when needed, with its own X session.

Battery is quite variable, but can be decent, depending on what you're doing. I use i3wm on mine, and powertop will show configuration settings that are non-optimal, as well as power-hog applications.