Actually responsive design would be your cure for losing the left sidebar - it would just tuck it down under the content.
While I love the idea of responsive design, I have not yet seen any discussion or studies on its effect on ad revenue. If you are using sidebar ads, that means in responsive design on, say, an iPhone, the ads get pushed down significantly under the content. To my mind, that could quite likely make CTR even less than it is now.
We were using the WP Touch plugin, but then our hosting company warned us it at the rate the traffic was spiking, we were using up way too much CPU with it. We ended up creating another theme, and using user agent detection to swap it out for mobile. And on the second theme, I used the 320x50 ad unit. It worked out pretty well (and no server issues) when we had a couple thousand simultaneous users.