What I don't really understand is why the touch is working with the stock rom but not with CM11. I mean it's the same kernel, i didn't flash anything new, just installed the CM11 zip using CWM. I might be totally wrong, but for me it just seems to be a driver issue. So there are new drivers for the touchscreen for the I9515, which are not used in the I9505 Roms, which seem to be the issue.
As the drivers seem to be included in the stock rom, isn't it possible to isolate the drivers and include it into the CM11? Don't you have access to all necessary files after rooting the phone? Or isn't it that easy?
PS: I'm not that much into the topic of kernels in general and Android-Programming, so I may be totally wrong