Have you seen Luke's tips and tricks on this video? Starts at 12:28
I do not think he says anything about shifting issues, or tricks. In any case, it seems that I might have messed up the shifter housing while installing it through the internal routing handlebar, because I have tried with an external cable and it did not take long until I got it right. After putting it back through the internal hose again, it does not shift properly (going up in gear, so releasing tension, which makes sense). So time to buy some decent shifter housing and try to install it without messing it up!
/Xtract