You can race it and the Liquigas team has. I think they use the new EVO now even on the cobbles as it has inherited some of the comfort features of the Synapse. As you note, they do have similar geometries - most road bikes do to an extent. I think the biggest difference a serious racer would notice is the taller head tube - around 20mm iirc. It would be just a bit harder to get the same low profile one could on a caad or supersix. Of course, if you don't need or can't handle that low of a profile then it's probably a moot point. Also, the supersix might be more laterally stiff, the Evo and Caad certainly are, which would be an issue to racers as well.

Like I mentioned, I own both caad and synapse and the differences are subtle. On anything less than around 40 miles it's hard to tell much of a difference and I've set PRs on both bikes on various segments - but that's mostly a function of improvements to the engine rather than the particular bike I was on.

Each bike has marketing points that appeal to different types of people. In the end, go with the bike that fits best and is fun to ride. I bought the caad over an alloy synapse because I just liked the ride better even though going by the marketing theory I should have bought a synapse. Later I got the synapse too (N+1 and all) mainly because I wanted something a bit better suited, to me at least, for longer rides.