I haven't read all of this thread but there is one thing I couldn't see mentioned and that's the size of the bit/iron is more important than the power of the iron.

When you apply the iron to the workpiece the workpiece cools the bit. If the bit has too little thermal mass the temperature will drop a lot and it's very unlikely the heating element in the iron can compensate and reheat the iron fast enough. An iron with twice the copper mass is usually better than one with twice the wattage (although obviously they go hand in hand to some extent).