Actually 7:20 is perfect. You should be able to finish your meal without having to rush and be able to go out on to the veranda or dock to watch Wishes. As was previously stated, even with a window table there will be people standing in front of the windows by the time the fireworks start. We have never been able to see from our window table so we quit even asking for one. They do dim the lights and pipe in the music both inside and outside.
By the way, Narcoossee's is a great restaurant. One of our favorites and some of the best meals we have ever had at Disney. We are seafood lovers.
Cheers and have fun!!!