I believe that the 68030 is the best compromise between compatibility and speed. Most games don't work with 040/060, so such a "high end" thing would not only be insanely expensive (way more logic and the overpriced CPU itself), but you'd always need a secondary machine with a slower processor. Those apps that really require some CPU horsepower run perfectly (and a lot faster) on emulation.
Remember that these accelerators were created in order to give a descent amount of memory to the Amiga for a reasonable price. Speed is not the target - it's mainly memory. The faster speed is a positive side-effect.