No. Who's Talking was awarded second place of the "Player Experience Category" of last year's Overwolf Guild Wars 2 App Challenge. The winners of that contest were picked by ArenaNet developers. So yes, they are okay with it.
Yes. Who's Talking works in fullscreen and windowed fullscreen. It does not work properly in windowed mode, but that's something we might change if there's demand for it.
Maybe. We're rendering an additional 3D projection on top of your game, so if your PC is already struggling to run GW2, and on top of that you're watching a Twitch stream on your second monitor, don't expect Who's Talking to run smoothly. But you'll most likely see Who's Talking having issues with the limited computing power, not GW2. (For the tech nerds: Overwolf windows are background processes, so when resources are tight, foreground applications like GW2 will have priority.)
To ensure that Who's Talking has enough computing power left, we strongly recommend to limit GW2's FPS at least to 60, because otherwise GW2 just takes all computing power it can get. On top of that, you can adjust Who's Talking's FPS in the "Performance" menu.
On top of all that, we put in a lot of development effort to make sure Who's Talking only uses resources if it actually has to.
Because it hasn't been asked yet. Feel free to contact us with any question you might have.