I now have a Kowa 664 so can answer my own question. The zoom eyepiece is sharp at 60x and I actually prefer it a little bit more than the MCII for the Nikons.Was I correct in thinking that the Kowa 660 scopes are sharp up to 60x? Or do they peter out above 50x? Not in terms of brightness but sharpness.
The EDIII scopes I have owned were tack sharp at 60x. That's my expectation for a 60mm scope. The Kowa 66mm should be brighter and have higher resolution but I have never used one.
The fixed 30x is also really good on that scope.