So.. Ferrari chief engine designer Lorenzo Sassi started work at Mercedes on Monday. He was
fired from Ferrari mid last year for trying out a concept that would have put the Ferrari engine level (if not slightly ahead) of the Mercedes engine on power delivery and output. It didn't materialise, so Sassi was fired. Typical Ferrari way of dealing with their staff. Sassi was a long time Ferrari employee and his knowledge on the Ferrari PU is second to none.
The last top ranked Ferrari guy who got fired and joined Mercedes was a one Aldo Costa. And look what he produced chassis wise for Mercedes.