I also went from cervelo to a s-works. I was on a 54cm p3. I am 5’11. I switched over to an XL transition, after getting a F.I.S.T. The difference between the large and XL is just the reach. The stack is the same. To give you an example. I was riding a M transition (new model) for IMAZ in april, but rode it slack. After getting a FIST I realized I like to ride at about 79 degrees, which put me smack on for the XL transition.
I am on a Large S-works. I am 6’1 with around a 32 inseam. I have to follow UCI regulations, so I can’t ride steep. Not sure if that will help at all. Oh, and for what its worth, my road bike is also a 56.