Hi @Marcymarc,
I heard this argument before and you are completely right. Buying a Pi to get ONLY this feature and using nothing else from rooExtend make no sense. This Roon Extension was made for rooExtend users 
My plan is to soon offer the rooHead calculation as a Web service that can be used by everybody without buying a Pi 
Sure you can build the equalizers by yourself if you are skilled enough. But you will loose some nice additional features rooHead offers.
- Easy to use in Roon (select headphone and go).
- Extended frequency range up to 96kHz without cutting off at 22kHz.
- Adjustable Bass Boost and Tilt to match personal taste and listening experience (explained in the Harman document above).
- Option for headroom management, so you can turn the convolution DSP on and off getting the same volume to better explore how the equalizer improves the sound.
Best DrCWO