I've smoked 15 lbs turkeys twice in my #4 and in both cases didn't reach 140 within 4 hours smoking at 275 and still here to talk about it, didn't kill me.
I don't think 15 lbs is really "pushing the envelope" too hard, as far as size. You are certainly right, though, about being temp-driven! I agree with Kari, that if you smoke a larger bird at oven temp (325, in a D-model), you should be able to get away with it. But, limiting to about 14 lbs, at lower temp (like 250, where I smoke mine) simply
reduces the chance of ill-effects. If you stay in the danger zone longer than 4 hours, it's certainly not a "sure deal" that you may grow some unwanted bacteria, but the odds go up dramatically, so why risk it?