The 416 is inherently hissy. Does anyone know the rough frequency of this hiss so I can notch it out in post?

I have two Sennheiser 416 mics and neither of them is inherently hissy. The problem that you're experiencing is almost certainly with the preamps providing the phantom power. I got a Sound Devices MP-1 preamp to link my 416 with my H4n and got beautiful clean sound.
These days I have a Sound Devices 664 and the only audible hiss on my recordings is from the source or the environment. I still use the 416 for external recording.

You might be better off increasing the pre amp gain before notching out frequencies. The area of hiss will be in the area of 5k to 8k which will harm the sound of your vocal recordings as that is also the area of clarity and intelligibility. Try the Triton Audio FetHead that will fix it for you. linkd is here: http://www.zenproaudio.com/tritonaudiofethead.aspx