Sunfish is really impressive work. From my (brief) understanding of Sunfish, the evaluation function is essentially equivalent to a hardcoded 1 layer network in Deep Pink.

You're right that everything else equal, a better evaluation function should lead to a better chess engine. However in practice I think better evaluation functions means slower evaluation function. So there's some really interesting trade-off there. I doubt humans evaluate more than a few thousand positions, so it seems like a slow but more accurate evaluation function could play chess pretty well