We just did it last night. We were there at 8pm and left at 8:15 and drove over to the Swan, Dolphin, Studios and then to stay under the bridge to watch Illuminations.
Honestly, if you could do it another night without taking the risk of a late airplane, slow rental car place, traffic, etc., I would do it another night. I think you may be cutting it a little too close, IMHO. It sure is worth doing, though, but preferably on another evening?