-are there real examples of otherwise unintelligent people being able to perform intricate calculations in specific domains?

I can't speak to their intelligence, but if you hang out with sports nerds for long enough you will encounter one with an encyclopediac grasp of it, play by play recall of numerous matches, nigh on eideitic recall of statistics, and sophisticated ability to relate and manipulate said statistics. Intellectual curiosity and intelligence may be linked but it's not a necessary link.