Thanks for sharing that link @cryptotem. The post and comments on it give in-depth detail about how steemit works in terms of steem power. It looks like there is no compound interest but we also can not rule the fact out that SP accumulates interest. It is clearly mentioned in FAQ. Instead of compound it might be simple interest.
RE: 4 Years From Today You'll Have Over $150,000 US - Worst Case Scenario - If You Keep Posting, Commenting and Curating Daily