Just upgrading the shifters probably won't give any increace in the feel / speed on the shifting, for this you will need to look at upgrading the deraillers and possibly the cables as well.

Being as you currently have a 2012 bike, you would need to look at a substantial upgrade to XT or XTR to get a noticeable difference, but any upgrade will have this added issue of needing to look at going 10 speed. Alivio / Acera are the lowest groupsets still on 9 speed, with everything else above being 10 speed.

Would follow MechBgon's advice on setup, and save your money for a new bike in a few years with better components when you have worn this one out.