We ate dinner at Fulton's Crab House before La Nouba last time. I can't remember the EXACT time but I do think it sounds like your reservation is cutting it a little close. Show starts at 9...seating probably begins around 8:15 or 8:30. If it's busy it could take an hour to get your food and then you have to eat. I can't be sure but it looooks like T-Rex is actually near Fulton's and I know La Nouba is on the West Side. So you have to factor in walking from one place to the other.
That is all hypothetical but if it were me I would probably try to move my reservation just a wee bit earlier.