Sorry man, those days of infinite budgets and major label greatness is gone with the downloads, especially considering metal acts.
Most probably the big time producers you mentioned are using the Kemper as a great tool to expand their armada of tools in reamping and for sure they are experimenting a lot with the Kemper or Axe or whatever else they feel handy. If they say that the new XYZ Studio compressor is the best since the invention of scrambled eggs, you can be sure it's a good tool, but they will not use that exclusively. Don't get into the illusion they track only with a Kemper.
I'm not a session guy, I don't need my perfected profiles to be available on my fingertips do lay down some tracks for a client in the most effective method. (cheap)
I strictly talk about live use.
Let's not try to compare in the room and recorded tones. The Kemper is reproducing the recorded tone, including the mike, the pre anything in the chain.
It can get pretty close to the recorded tone of an amp, but it cannot and never replicate the moving air and the way a pair of human ears capturing that particular sound. That's why it's a perfect tool for engineers, to do the multi sampling (profiling is such a hip word) and replicate the sound of a recorded amp through Neve, API, SSL or whatever pre with the best mikes available in a shed in the middle of the forest.
It's a great tool, but don't tell me it can come anything close to a tube amp blaring next to you. It simply does not.
Peace