In answer to the original question, my experience in the Caymans has been that the dive operators do not limit divers depths at all based on level of certification. If the first AM wall dive is planned for 70, 80 or 100 feet, that's the depth you will be permitted to go. The conditions on the Brac are generally so good that there is little appreciable sense of depth and an 80 or 90 foot dive is no more difficult than 60 feet. By all means,Let the DM know that you haven't been on a deep dive on a wall previously. The DM will undoubtedly keep you close to him/her on your first few dives to check that your basic skills are OK and that you don't do anything stupid. You will be asked to let the DM know when you are at 1000 PSI so that they can direct you to the mooring line. On your first few deep dives, you will probably suck more air than more experienced divers, no big deal.
My own experience with AOW was a positive one in that the course emphasized skills and details that had only been glanced over at my original OW course e.g. navigation, and advanced buoyancy. The AOW if worth doing even if just to refresh skills and didactics.
My comment would be that a lot of the Brac's first morning dives are wall dives which tend to be deeper, but the staff is often very good about picking sites that are a bit shallower at the beginning of the week...places like "East Chute" where it is 50fsw to the sand under the boat.
