Re my comments above: now Monday afternoon....back from Dealer re 5th gear 'grinding' noise.
Now here's the simple, no-charge fix for "my" fifth gear problem. It appears that I had a "brain fade" on the weekend, and apparently somehow "created" my own [not-being-able-to-shift-to-fifth] problem. I apparently *forced* the shifter too far to the right, and the reverse lockout "spoke to me" with the grinding noise.
That first time it happened (last Friday), I guess I didn't find fifth easily, and moved too far right and forced my own problem. At the time, I had only 1100 miles, and the transmission is not even broken-in yet. And, I repeated this maneuver several times after, all with grinding noise.....all too far over. No, there was no beer in the Z06.
Yes, it was somewhat dumb on my part, but maybe this could be common to some others too who have experienced this phenomenon.
Embarassing fix....but cheap.