Isn't it plausible that a clever species such as our own might need less pain, precisely because we are capable of intelligently working out what is good for us, and what damaging events we should avoid? Isn't it plausible that an unintelligent species might need a massive wallop of pain, to drive home a lesson that we can learn with less powerful inducement?

I don't think it's plausible at all. The smarter a species is, the more scope it has to go wrong outside of basic actions, the more urges it has unconnected to basic eating and mating. Even a smart species can't directly calculate the fitness of every action. (Heck, I couldn't calculate the fitness of eating right versus eating wrong.) At best, I'd say intelligence has an indeterminate relationship, and if I were allowed to appeal to humans for evidence, I'd point out all sorts of utility-raising and fitness-lowering behaviors like condoms or memes where the intelligence has rather backfired on the genes.