AC Unity is a stutterfest on PC, don't worry about that its the problem of the game's optimization. But FC4 shouldn't lag on your card, you would experience only slight stutter like I do on my 280x toxic, but the FPS are actually quite healthy. I used to notice stutter only while driving.Try this easy slolution, it worked for me:
http://forums.ubi.com/showthread.php/954533-EASY-FIX-Stutter-Frame-Skipping-Far-Cry-4!
Also, try switching the game to ultra, it actually reduces stutter especially if you have an i7 (my expericnce). Increasing the demand on the system makes all the cores work to its max thus reducing stutter that is what I think.