I notice a difference, both against the wind and in descents. It's not a huge one, but it's there.
The thing about drops, though, is that they may also end up delivering more power to the pedals. I'm not a racer, but even I can kick it up a gear when I'm in the drops. So it's not entirely an aero question. Considering that both factors are relevant to headwinds, I'd say get the drop bars.