User blog comment:MarineroLelouch/Power Ranking and Classes./@comment-92.96.32.189-20140215151218/@comment-24066070-20140215184751

It was never shown just how much Sting's power increased from the so-called boost. As Nick said, beating someone who has his guard down doesn't prove Sting is strong.

Besides, Natsu gets a powerboost every 5 minutes, so even if Sting had overtaken Natsu in terms of power (this is next to impossible), then Natsu would top that right away with another nakama speech.