My wife and I just ate at Sci Fi last Thursday with no ADR's and it was around 4:00-4:30 or so in the afternoon. Obviously just the two of us and they quoted us a 30-40 minute wait. We ended up just sitting in the Writer's Shop for about 15 minutes and were seated in a car (as hitchhikers)

The restaurant was about 1/3rd full when we were seated. Though it began to pick up as we left. Dinner hours of course.
We do this a lot even at peak times for the parks and very rarely do we (as two) wait longer than 30 minutes with no ADR's. Now, they have told us no straight up and we were good with that.
I know they have the tables at the back of the restaurant though we have never been asked to sit there. We have always been in a car as walk ups. We've probably eaten there at least 12+ times this year alone with no ADR's.
If you have your heart set on it, I would for sure try to get ADR's if you can. Other wise, try walking up at non-traditional dinning times.