More nodes do make a big impact cause you are calculating stuff rather then just reading it from a map. But firefly itself is so slow and inefficient in handling maps and geometry that the difference extra nodes make isnt very noticible. So you should go for what looks best and not worry about rendertimes or node counts that much IMO. If speed is what you want, then there are a lot better renderers you can use then firefly.

Also note that the large number of nodes people use is not because the creators want it that way. More like they have no choice in the matter. A lot of the nodes are simple math nodes that are scaling stuff, isolating stuff, adding noise or reacting to scene data like light angles or intensity and so on. There is no way to replace them with simpler nodes, because they are already working at a very low level. If you replace them, you lose the detail they are adding to the tree.