I have some FreeSync questions

So, I have an Asus MG279Q. Basicly, 27'', 1440p, 144hz, IPS, FreeSync range from 35-90Hz.

When I first gamed at 144Hz I was blown away by how better it looks compared to 60Hz. Never tried FreeSync tho. Will it feel as much of an awesome thing as the jump from 60hz to 144hz?

Is it really that worth it? I'm interested in the RX Vega 56 and one of the reasons might be this, otherwise I'd go for a 1080 or wait for Nvidia Volta.

Also, is there a way to change the refresh rate of the monitor on a per-game basis? So far it seems like the only way to activate FreeSync is if I manually go to resolution settings and change refresh rate to 90hz instead of 144hz. I'd like to have CS:GO, League of Legends and Overwatch running at 144hz but wouldn't mind having a 90Hz "lock" on the rest of the games since with a Vega 56 I wouldn't probably reach that high of a framerate either way.