The 56 Synapse is only 4mm longer in the reach than the 54, whereas it is 20mm taller at the front end. So you could always shorten the 56 stem a little to get the fit right.

I'm also going to suggest you look at the Fuji Gran Fondo range.
Their 54 (545mm ETT) is 12mm taller at the front end, and 9mm shorter in the reach than the Synapse 54.
Their 56 is 12mm taller, and 6mm shorter than the 56 Synapse.