On Saturday we tried adjusting the cables with the method from
@Jajodu ,but with the forge short shifter installed.
Turns out the shorter throw makes it impossible to do the proper adjustment because the plastic holes do not match up anymore. The hole on the side of the shifter assembly can still be used with an Allen key, the one on the back of it does not line up anymore. Nevertheless we tried keeping it in position as good as we could and then released the pressure off the cables from within the engine bay. We noticed a small enhancement. But it's nothing that can be done real quick. You need time to find a proper throw adjustment of the forge shifter which works well enough with the adjusted cable lengths.
We now just need some kind of tool to keep the shifter locked in position without utilizing the holes, this would make the process a whole lot easier.