In theory this sounds like a good idea, but it's really not (we did it last year).
As mentioned before, you might not end up with a window seat (we didn't).
There's no perfect solution for timing - you may end up getting served quickly and then just dragging things out waiting at your table for the show.
They do pipe the sound for the show into the restaurant, so that's nice, but watching it from inside just isn't the same as outside.
All in all, more hassle than it was a benefit. If it doesn't mean that much to you and it works out for you, great. We'd seen Illuminations outside many times so it wasn't a huge deal that we weren't at a window, but if it's your only opportunity, I'd advise to reconsider.