That means 15% of the reward pool, not interest on the SP itself.

15% would be 150 SP per year per 1000 Steem powered up. Using the 500 SP delegated I mentioned before, that would come out to 6.25 SP growth monthly if it was 15%, not taking into account the compounding affect. As shown in the case of the delegation I have received, it has only grown 1.348 since I received it on 10/06.