I found it interesting that, according to the BBC report, it's not the Horner situation per se that's causing Newey to look elsewhere; rather, it's the toxic environment within the team that has resulted from the investigation. Sounds to me like a lot of team personnel have picked sides and are then at loggerheads, which is upsetting the tranquil environment Adrian prefers to work in.
Mind you, if he wants to move because the internal politics are not to his liking, heading to Ferrari is probably the worst choice!