I don't mean to be disrespectful to your concerns, but it seems to me 
that you're getting all wound up in a non-issue.  

As many knowledgeable people have pointed out, msg is a naturally 
occurring substance in a lot, if not most, foods.  When food 
manufacturers add it to a preparation, they do so because it's a 
known flavor enhancer. 

Your wife's theory, that MSG is added to food to stimulate appetite, 
may well be true.  But I don't believe it's ALWAYS the reason it's 
added.  People are (largely, for the most part) in charge of their 
own appetites. 

You don't know much about cats, do you? 

Cats will Take Advantage of You.  Resign yourself:  you will never  
understand a cat.  Their tastes are whimsical.  

I also suspect, though it's been a while since I've checked ingredients 
on commercial cat food, that there are much more stringent requirements 
on pet food additives than human.  

See, the FDA has this stupid idea that human beings have the intelligence 
to look out after their own interests.  