I fish very small streams and frankly I don't really target 20 inc fish necessarily... If I did and I used a larger rod and heavier line, most of my catches (8-14 inch bass) would just get drug in on the top on my heavy line.
It's like fishing the Niangua with big rapalas for big trout.. Yes, if you hook one you'll have a much easier fight, but meantime your dragging in stockers left and right accross the surface and that gets a bit boring.
4lb p-line has been doing a great job for me on an UL.. But then again I may be "willfully ignorant" and am just enjoying my good time either way.