Well, ok, what happened to me with the expo was that my old radio is a JR x-347. I just bought a new Polk Traker II radio a bit ago, and was moving planes from the JR to the Polk.
"Hmmm, this touchy combat plane uses 50% expo on ailerons and elevator, ok, dial in 50% on the Polk."
Well, as it happens (and I knew about) JR and Futaba work backwards from each other with Expo. In JR-speak 50% expo makes the center soft, in Futaba-speak it's -50%. Notice that little, itty bitty "-" out there in front. Well, Polk happens to follow the Futaba way, which I didn't notice in the instructions. It was a very exciting flight, let me tell you.
Using the expo to overcome a sluggish plane might work, yeah. I haven't seen it done though.
I did talk to someone this weekend who intentionally sets his combat planes with expo to make them sensitive around center. He feels it allows him to react faster, but hold turn radius more accurately. I can believe that.