How about this... Dorel used to use overhead shields for their seats. When they stopped making them, they used the same mold for their seats, just didn't add the shield. Well it just so happened that on that mold there was an indentation for where the overhead shield used to attach. There have been claims that children's heads have impacted that indentation, causing injury.
If you remove a structural part of the seat, there is no knowing how it will perform. There could be a bump, or indentation, or piece of metal, or who knows what that your child's head will impact in a collision. The back of the Guide65 was not designed to be impacted by a child's head because they made it to have headwings. Also, removing the headrest would likely reduce how long you can use it. For rear-facing the child's head must not go above the top of the headwings, and forward-facing the child's ears must not go above. If you remove the headwings, the shell of the seat is much shorter, therefore reducing how long you would be able to use the seat.