I play CS GO too, actually a pro player mind you (Global Elite - Lanner)
GPU doesn't matter much, Source engine is more CPU intensive, so i5 would be better, I could give it a try later today, I figure we can get around 100fps on a decent resolution above with medium-ish settings. But bare in mind, running that much fps can defo heat up your tablet, plus CS GO is a game to run at over 200fps to fully enjoy it. Please before people complain that eyes can't see the difference between anything above 60fps, you can feel the movement is different and as a gamer who plays on a 144hz monitor, you can
On a 144Hz monitor at 144FPS, you will benefit over a 60Hz monitor at 60FPS in several ways. However, playing at 200FPS on a maximum of 60Hz Surface Pro 3 is completely pointless - a waste of power.