I think you'd be surprised what you can get away with, especially fishing topwater at night. With low light, and a noisy fly to draw their attention away from anything besides the meal and you should be fine with 12lb but I wouldn't hesitate to go heavier if I was worried about losing big fish around heavy cover. As long as it's not too stiff to deaden the action of the fly I think you'll be ok, and I've never seen any 12lb line that would come close to being too stiff. I use a lot of 17 and 20lb mono/fluoro when fishing bigger flies. I think there's a relationship between fly size and tippet size as far as drawing the attention away from one and towards the other. 2x on a size 14 makes for a knot thats clost to the size of the head, possible the body of the fly. 20lb on a size 1/0 makes for a knot maybe half the size of the eyes on the fly. Just something to consider when choosing leaders where drift isn't a factor.