Has any tried using an external teleconverter in addition to the built in teleconverter on the new Nikon Z 600mm f/4 lens? I am wondering if adding an Z TC 1.4 will give sharp images with the extra reach obtained, which should be 1176 mm by my calculation. Not often needed, but you never know when you might want it. I am assuming that the wide open f stop will go from 5.6 to 8. Thanks to anyone who can provide any information in this regard.