1) Not sure what else I can add, as I gave the link to monitors.
2) MIXING refresh rates may be an issue and cause STUTTERING issues and/or screen tearing. It was an issue, however not certain if mixing 144Hz + 60Hz works fine now or not.
Pretty sure a game running "WINDOWED MODE" would be a problem since both screens should be running the same frequency for that to work properly. Not sure if there's some combination that works fine (such as game running FULLSCREEN on the main Windows screen at 144Hz, and the 60Hz just being the Extended screen?).
*I admit I'm not sure how things currently are. You would have to TEST things yourself.
3) Just FYI, but MOST modern, demanding games dip below 144FPS. Look through the list of games, and note that many games dip under 75% of the AVERAGE value that's shown. In fact, I've had to TWEAK the settings for several games to maintain a solid 60FPS.
Roughly speaking, 80FPS average is needed to have minimal dips below 60FPS though every game varies. Some games can frequently drop below 50% of the average value.
https://www.techpowerup.com/reviews/ASUS/GTX_1080_STRIX/16.html
Since you won't have GSYNC you'll have to use HALF ADAPTIVE VSYNC (72FPS on 144Hz screen) if you want to avoid screen tearing, and in addition you'd also need to TWEAK the game settings for same games to get a stable 72FPS. The ONLY time I recommend normal VSYNC ON is if you rarely drop below 144FPS.
Most games have settings that can be lowered with minimal visual impact. You have to experiment but you should ALWAYS BE TWEAKING TOWARDS A SPECIFIC GOAL (not just setting to "Ultra" and accepting whatever FPS and game smoothness you get from that).
VSYNC OFF is ideal for many people if the amount of screen tearing is minimal. It varies by the game, and your settings, so it's not possible to predict exactly what will work. You just have to experiment.
Sorry if that's confusing. Again, you do NOT want to use normal VSYNC if you frequently drop below the target (i.e. 144FPS) because that creates a synch mis-match which creates stuttering (stuttering can exist for other reasons as well).
Huh? Why? Because the MONITOR (non-GSYNC) updates at 144x per second according to what's in the buffer. However, VSYNC will not mix different frames (to avoid screen tearing which is causes by mixing the content of more than one frame).
So if the new FRAME isn't created in time (under 1/144th of a second) the buffer will not be updated with the new content. What WILL happen is the monitor will instead draw the EXACT SAME FRAME again.
The final result over a period of one second is that you end up mixing MULTIPLES of 1/144th of a second depending. Some may be 1/144th exactly, some may be 2/144th and so on.
The result is called "micro-stuttering" and shows up as a quick stuttering especially when panning quickly.