For what it's worth I've never had any issues with S-Works cranks and rings on my Tarmac. They were fine on the Shiv when I had them too but then Di2 will shift the shit out of anything.
I've got a near new S-Works Shiv crankset in a box at home as well as 7800 DA cranks. Probably should get rid of them but think they might end up on an audax bike if I build one.
Read the notes.
For what it's worth I've never had any issues with S-Works cranks and rings on my Tarmac. They were fine on the Shiv when I had them too but then Di2 will shift the shit out of anything.