It won't be the money best spent, in my opinion. I have both Alivio and Deore rear mechs on my 2 bikes and can't feel any difference in shifting. Alivio is more massive and heavy, but, from my experience, lasts longer than Deore. Same stands for combo shifters. By the way, shifters, for all I know, are sold in pairs only, right and left one together, which adds to the expense. Replacing cables/housings with new ones will probably give you more improvement for fraction of the cost. Also, if you are experiencing problems with shifting, it would be a good idea to check/straighten the derailleur hanger. You may combine this with some other repairs/purchases at your LBS, so you may even get the work done for free or almost for free, as it takes just couple of minutes. It is possible to do at home as well, but hard to match precision of the device a good LBS keeps for this purpose.