Originally Posted by BruceBrown
I wouldn't bother with upgrading your front derailleur. I have XT, Deore, LX, Alivio and even Altus front derailleurs on all the bikes currently hanging in my garage. They all work fine and the performance difference is pretty much minimal if any. XT's are great front derailleurs, but the LX and Deore are flawless as well if you have them dialed in correctly and keep things lubed properly.
My advice is to only spend the money on a new rear derailleur. Again, be it LX, Deore or XT you will be satisified. See what you can find on sale or the best price and go that route.
BB
My experience with Alivio, Deore, and XT generally agrees with Bruce. I must say that I find the XT rear shifts significantly smoother than the Alivio even at optimal adjustment. The difference between the Deore and the XT is more subtle but still there. The XT is also better built than the Deore (i.e. should last longer). I find the Deore front derailleur to be significantly crisper than the Alivio (or lower). I'm not sure I feel any difference in shift quality between the Deore and XT in front (although again the XT front is a nicer piece of machinery and should last longer).
Just my 2 cts.