In high ambient noise, earplugs are good. They're kind of like sunglasses for the ears. Sunglasses are good in high ambient light but seriously cut down what you can see in low light. Earplugs are similar wrt to noise. I wear earplugs on my motorcycles where wind noise drowns out low level sounds. But not on bicycles where the wind noise isn't as high, especially when going slowly up hill etc..

I have the slipstreamz (both types, the ear covers and the spoilers) but haven't yet played with them enough to conclusively determine their efficacy. So far, I find that the ear covers do cut down wind noise significantly, but not enough, IMO.