Full sun may be the ideal for many varieties, but I have grown tomatoes in similar conditions with excellent results without crop reduction. As long as the temps are warm enough tomatoes seem to do ok with less sun.
That has been my experience.

A half dozen years ago, I lived just to the north of you in Buford. Some of my containers only got 4 hours or less of direct sunshine, but they got a lot of "indirect/reflected" light. I didn't get quite as much production as I would have liked, but I got a lot of tomatoes.

I have been doing with 4 to 5 hours of DIRECT sun.
Direct sun is not the only source of light for plants for their photosynthesis ; Indirect and defused light also counts. Plants can get quite a bit of light even in a day with overcast skies, as much as under grow lights.

In my case, my tomato plants in different location get 4 to 5 hours of direct sun BUT with 14 to 16 hours of day length from may to September they get plenty of indirect light. Shade is not equal darkness.

BTW: I have gardened In North Fulton/Alpharetta for few years with about 6 hours of direct light. Then again with a lot of defused light. As mentioned, if your temperatures are high ( like in North Atlanta in 90s and over 100F for days ) actually 6 hour sun is better than say 12 hours.

"defused light also counts."

seysonn nailed it. There is a lot of misunderstand about diffused light. Diffused light is aklmost always better and one of the principal benefits of using poly film greenhouse plastic. The only way I have worked out what my plants get is with a light meter. A cloudy day does better than my lights until an hour before sunset. That has become an important fact as I supplement my straggling plants to catch up partially in my late fall growout trial.

"even in a day with overcast skies, as much as under grow lights. "

Bingo again. Except to clarify we are talking about low power grow lights. High intensity lights (professional) and some new very expensive LED lights can do much better.

PC

Thank you everyone for the replies!

Has anyone found any specific varieties that seem to do better in the shade than others? For instance, I've heard some people say that Stupice or Bloody Butcher may be good ones to try with lower light.

My grapes & cherries got 4 hours of sun a day and did just fine. I have read that cherries/grapes do better with less sun than globe/beefsteak.

Excluding this year I believe that less than 6 hours per day would be normal in my PNW area.