Tone is incredibly subjective and one man’s better is another man’s horrible.
I have absolutely no doubt that the Kemper can produce high gain tones that are as good as anything out there. I also have no doubt the AxeFX/Helix/QC can all produce similarly awesome tones. Which is better is simply personal taste. The main reasons to choose one over the other are features and workflow.
The grass always seems greener on the other side.
Just as an example, the most hyped piece of gear on the market just now is the Quad Cortex. So many people have been saying that it captures amps more accurately than the Kemper for both tone and feel/response. However, I watched a blind test video the other day (Tone Wars) where three different guitarists (all high gain players) each profiled an amp of their choice on both the KPA and QC. Then they played each along with a song and in isolation. At the beginning they were all expecting to see the new Kemper Killer reign supreme. However, when the scores were tallied an the amps revealed, all three had chosen the KPA as being closest in sound and feel to the real amp. These were all high gain players testing high gain sounds.
As I was watching the video, I kept choosing the same amp as they did and this became even more so the longer I had to listen. The QC became more and more tiring and ice picky the longer I listened.
Basically, the KPA can do high gain as well as anything its just a matter of personal taste which system suits your needs best. Remember, we are all guitarists and we are all suckers for new gear but in reality the gear we already have is almost always capable of much more than WE are.