That’s a very interesting point.  John could be taking the Passion story from the synoptics and just making Jesus stronger, more certain, more divine, less human–but still holding on to certain key elements and phrasings.  A process we see happening progressively from Mark onwards, that reaches its apogee in John (not counting much later stories, like the one written by that German nun that inspired Mel Gibson). 

However, all the gospel authors had earlier sources.  There were probably written accounts of the Passion that are now lost.  This is where textual analysis runs into a roadblock–we don’t have all the texts–we can’t know what is original to each gospel.  We can assume many of the things Jesus is quoted as saying were well-known to Christians before any of the gospels were written.  Which is not proof that Jesus really said them, and that goes for all well-known sayings of figures from ancient history.  Traditions take on a life of their own.  John is reacting to earlier versions of the same stories.  He’s trying to make them match up with the Jesus that lives in his imagination.  A divine being, the incarnate word of God, perfect in every respect.  Incapable of fear or doubt.  

I think it’s likely Jesus talking about his coming sacrifice as a cup he must drink from is something that was well-established before Mark wrote his gospel, and the phrasing and context would vary from account to account.  We have strong reason to question the story of what he said in the garden when he prayed–since if everyone was asleep, who could have heard him?  It’s likely to be an extrapolation from surrounding events–an extension of the metaphor from the Last Supper (I see no reason at all to doubt they had a meal together at the Passover, and Jesus would have been saying strange portentous enigmatic things, because he did that a lot.) 

I would go further and say Jesus was probably known to use phrases like that.  He was thinking about the possibility of martyrdom, certainly after the beheading of his teacher John the Baptist.  So it’s just about how each successive writer chooses to incorporate the basic phrase and concept into the story.  It was there from the start, but the original form is unknowable.