I would expect image quality to be "stunning" even with the extender in place. I know it is just me...but I do not "get" a lens of this configuration at this cost factor. I guess it gives pros good versatility in certain situations.
...but at that cost with an extender switch..sounds like the lens could be open to some serious maintenance situations down the road. Not trying to be negative...just standing back and scratching my head, based on what I know about photography. It is interesting, to say the least.