It's entirely possible that the god's power is not even proportional to faith in it, particularly in Gensokyo. I would wager that the god is inherently powerful, and as long as they have some faith they can use their abilities. Instead, a need for a large base of faith is primarily for their influence on the population, allowing them to act for that population. A god you don't believe in can't really help you as that god. A god many people believe in grants the god influence over those people and allows for direct interaction. This is fairly obviously Kanako's goal, rather than accumulating sheer power. Having sheer power proportional to faith makes no sense. Rather, the mechanisms for faith in Touhou follow heuristics that are very similar to how they act in real life, but are applied in a world where said gods physically exist and exert influence. It's good stuff.