On Tuesday I was stood in the Specialized Concept store looking at the Ultralight version of this. Very very nice, but perhaps a little too aggressive a riding position for me.
I normally ride a 54 (Trek Domane) with rather relaxed geometry, and I did wonder if I might fit a Tarmac 56 as the stack height would be around the same, but was worried that the saddle would be too low. However, when I checked the geometry I found the seat tube length of the 56 was exactly the same as my 54 Domane, so perhaps...
Just have the longer reach to deal with then, and a shorter stem could solve that.
Not sure about the disc version, it does give up a lot of the weight benefits (seems to be around 600g in Dura Ace) and I rarely descend steep gradients in dodgy conditions.
Geoff