Replacing a shifter is a novice level repair. Visit parktool.com. An XT shifter will be "better" in the sense that it should be lighter and is made of "better" materials. Quantifying the cost vs. gain is pretty hard to say. Esp. when I haven't ever made that move.
You will have a better shifter and the fact that it is mated to a Deore derailleur does nothing to change that. A pair of Armani pants matched with a set of Fruit of the Loom undies doesn't diminish the pants nor the unders for that matter.
I have a set of cheapie ef29 shifters mated to a brand new LX derailleur on one of my bikes. It shifts great and I am happy. Despite the fact that my shifters are inexpensive. It shifts perfectly for me.