I am not a mechanic so I will limit my comments to my own experiences. I had very similar questions to you when i started out. I always figured that putting an XT rear derailleur on one of my mountain bikes would really transform it. It didn't.

I have used acera, alvio, and now XT on my two mountain bikes. What i can tell you that they all shift exactly the same when adjusted correctly. That is the key. I have had my XT derailleur shift poorly when it was adjusted poorly.

I have had my Acera derailleur shift perfectly, when adjusted perfectly. No doubt the XT derailleur is of better quality and it will probably last longer. NOt to mention i thing the pulleys will last longer. But in general i think the differences in component levels are overstated in most cases.