I tend to think that "optimum weight," at least for anybody who isn't a professional or competitive cyclist, should be determined by something other than climbing. Strive for a weight that feels like a healthy weight for your body type and age, that makes you feel strong and athletic, and that you can realistically maintain. If you consistently do both cardio, including hill training, and strength building work, then your climbing will take care of itself. If it helps, I know a lot of really good climbers who aren't necessarily thin. I also know that at any given weight, my climbing ability had more to do with the amount of hill training I was doing than my actual weight.