As you know mate I also have a Synapse mine has the 105 cassette in 11-28 this is great for really hilly rides I only ever get in the 50-11 when going down long hills and conversely only ever need the 34-28 for going up very steep hills, as most of my rides are on flattish terrain I find it very hard to get a good cadence with this cassette.
Consequently I am in the process of getting a 12-25 (12,13,14,15,16,17,19,21,23,25) 105 cassette which should smooth my cadence out no end.
I will then be able to swap cassette (and chain) depending on the ride I am doing, I did this on my previous bike and it was only a 5-10 minute job.