We have had multiple 7:15 ADRs for 9:00 show, and it was perfect timing.
We asked for a patio table, and got one each time (some views are better than others, but never had a bad one).  When we asked for the patio, they said they will try.  If you don't get a patio, they do have a private viewing area you can go to for Illuminations, then come  back to your table.  So, still worth it, even if you don't get a patio.
We were seated around 7:30 each time.  Ordered appetizers and dinner.  Finished dinner entrees around 8:30-8:45, and ordered desserts.  Desserts came AFTER Illuminations finished, which was PERFECT.  We ate dessert while the park emptied.  After we finished desserts and coffee (no one rushed us out), we had a relaxing stroll out.
The latest R&C ADR is 8:00 (or maybe 7:55 technically).  They KNOW people will not get up to leave after 8:30, so they plan accordingly.  If you finish eating at 8:30 or later, there won't be anyone waiting for your table. 
Timing of things change, of course, if Illuminations is not at 9:00.  I don't know if they extend R&C hours when Illuminations is later.
Pro tip: we used bus service to POR.  Instead of walking all the way to the front of the park and busses, we realized it would be a MUCH shorter walk from R&C to the friendship boat, and then walk to the HS busses.  Maybe took a bit longer, but much more relaxing, and we weren't in a hurry to get back.  We checked the HS hours first, to make sure busses would still be running (HS was open until 10, so we knew busses would go until 11, at least).