I'm a KPA user and I think this is massive overkill personally. The Kemper effects are good quality to my ears, sure they don't have the same flexibility as the Axe stuff, but is it really worth $1400 just to run parallel effects and get the few extras you don't get in the KPA? Not to me.
I guess if you run a valve amp separately as well as owning a Kemper it becomes more attractive as you could set it up to run with either which would be handy.
If it was like the G system where you could separate out the brain and the switches then I'd love the idea of the two together. But as it stands, to run the FX8 in 4CM with the Kemper would negate the big benefit I find with the KPA which is ease of setup.