When your games aren't the main focus of the monitor, it means your PC isn't paying full attention to it, just the bare minimum to make it keep working, that's why it goes to very low fps.
When you ALT+TAB back again, it will regain it's speed/focus. That's way you can see the frames going up again.
It's probably you overthinking about it. Don't worry.
And yes, i believe it's a worth upgrade since a 1080 is such an overkill for just 1080/1440p.
With that AND G-Sync, you'll have a great image quality. If you can afford, go for it.