All I can tell you is observations from my two neighbors.   One has Gerringhoff disc chopper and the other has Harvest tech.   The Gerringhoff guy uses a red DMI disc chisel after the combine does its job and the h-tech guy uses a new Will-Rich disc chisel after combine.

The harvest tech neighbors feilds are more black than the gerringhoff when done.  Good or bad it is what it is.

Personally Im not a big fan of lawn mowers under heads.  We ran two K&M roto choppers for ten years......Seventeen years ago.   They worked fine.   Only problem was juicy stocks would produce a fine SAP like substance that would navigate its way to the rotary screen air intake.   Worst night was every half mile shutting combine down to clean the screen so the engine would not overheat.  At the time, JD (were running 9600's) was VERY against using these choppers.........Found out why after two reversers crashed on both combines within 10 minutes of each other.  Had to drive 2.5 hours on hwy 212 (passing 4 JD dealers) to the warehouse to get them.   I think they solved the reverser problem nowadays.

Never had problems with sap on 1680 or 1688......could be air intake location? Or the bt corn that came after the red bines were traed for green that was the problem ......I don't know.

All chopping heads will take power from the combine.

Looked at a new gerringhoff 12-30 head this past summer..... we could have bought 4.5 30' stalk choppers for the same money!

Good Luck