I've been looking at doing the same thing. I have sourced some taller bars from LamontSanfurd. At the risk of sounding stupid, I'll ask anyway - how do you know when you've hit the limit for cable lengths? I'm sure there is more to it than just making sure you have enough slack to turn the bars.
I have a sidecar set up as well and think that the taller ones will make turning easier, along with a little more of a comfortable riding position.