The 100MHz increase is not worth the price they ask for it. You will never ever notice that difference under any circumstances. I have a 3930K in my desktop. It's an unlocked chip meaning I can increase or decrease its clockrate to anything I want.
If I increase it from 3.2GHz (which it ships at) to 3.3GHz there is no noticeable difference in performance. Even encoding MP3's using dbpoweramp (A well known transcoder) we are talking an album conversion from FLAC to 320 CBR MP3's going from 1 minute 32.5 seconds to 1 minute 32.1 seconds. The difference is that insignificant.
So in my opinion if you intend to get an extra BTO option restrict it to the SSD, Memory or Accessories. The CPU upgrade, for an extra 100MHz? totally not worth it.