I am certainly looking forward to trying this accelerator in my 600, and I certainly do not expect it to be 100% compatible with any existing CPUs, although it would be nice if it was. To get a major speed bump, you need to accept some compatibility problems, like it was the case when 040/060 Amigas came out. Much of the old stuff did not work any longer and had to be rewritten/patched. Although maybe not to begin with but eventually, I would close to 100% compatibility with the most common SW like WB 3.1-3.9, libraries and system friendly software.
This being said, some compatibility mode would be nice, for example which could be activated in runtime when running WHDLoad which would disable some of the new fancy stuff. Although this would slow down the CPU, WHDLoad slaves generally don't require a massively fast CPU anyway.