This is a great question to ask and also a very hard one to answer. The instinct is to say hell yes, but, the reality is probably no. I ask myself this whenever I start thinking of adding something new, what will the benefit be. Merely affording it via regular old economics (are your profits greater than your costs) does not answer this question.
It's a hard question because your always learning. Example: I can say I am creating a much greater variety of images with lets say my 24-70 2.8. But, I have now owned that lens for just over 1.5 years. The addition of that lens alone did not improve my photos though, it was the time spent learning the lens, finding it's sweet spots, etc, etc. Which was a process I am kind of going through again due to moving from crop to full frame! The 24-70 works in a different way now, and I have to learn it! Same for my 70-200, for a while that lens sat in my bag gettinhg used rarely because its a bit too long on a crop sensor unless your pushing range. But its a totally new lens to me now on FF. So in that sense, the gear isn't really the thing that makes things get better --- it's the time spent with it, learning it, using it, experimenting, taking it on the job --- that's what really makes a photo improve.
Bodies --- there are some things that I really just wouldn't be able to do without the ISO capabilities of the mk3. Is the quality of photo proportional though? No --- but, along these lines there is an intangible benefit --- that being confidence. Knowing that I can tackle any situation and get the shot, that's a hard one to quantify. Did I get that job because I am just so awesome how could they go with someone else? Or, does that little nugget in the back of the mind, that knowing I can handle anything - how does that factor into the in person meetings? I know if I was still only using my 7D, I probably would not have approached meetings with such confidence, I'd be worried because I know how shots look on the 7d above 2500 ISO. Yes, I have a set of alien bee's, but, knowing that I can push to 12,800 in the reception and get great natural lighting shots versus having to use lighting... how does that factor in?
Good Q...and very informative replies thus far!