One thing I forgot to mention is that strap mounted quieting devices like Slipstreamz and Cat Ears only work when they're aligned in front of the ear and into the wind, so for example, if you ride in a low position that angles the helmet straps more acutely in relation to the wind direction (e.g. in the drops), the devices won't be as square to the wind as if your head was more upright, and so the wind will come in over the top of the device onto the ear, making noise.

More upright positions, then, see the quieting devices perform better.

It's not a big deal, but being aware that they cannot work all the time (because of head movement and position changes) may recommend a different type of device if eliminating as much wind noise as possible is the imperative.