In most cases KWD is a major factor with Google. I've seen sufficient evidence that too high a KWD can hurt rankings. It isn't a "penalty" in the sense you drop off the radar. However, beyond a certain point increasing KWD decreases rank. I would suspect that the algo uses a sliding scale based on page size. 15% KWD can easily happen without SEO on a 100 word page. However, on a 2,000 word page 15% KWD is mighty suspicious.