I asked many people what is the defense cap in naxxramas i got all sorts of response ranging from 540 to 600 but most of the people tell me it's 540 so i assume thats it, so now i have 543 will it do any good if i keep raising it, or i can go with some stamina enchants instead like +275 hp on chest ?
Excess defense only works towards higher avoidance, which is not your main focus because this is WotLK. Once you get 540, you should focus everything on effective health - stamina and armor.