I have the Amira and Synapse (both Ultegra). I bought the Amira first and consider it a mistake (an expensive one). I too found the position quite comfortable (I'm also reasonably flexible although not through yoga). But what I found was that the bike was very stiff and therefore quite a jarring ride. I felt every single bump and riding on a rougher bitumen surface was torture. In comparison the Synapse is very smooth - if I didn't know better I would swear it had springs. I can ride a lot longer with much less fatigue on the Synapse than on the Amira. I find both bikes very responsive although the Amira is marginally more responsive downhill. Ironically, I am noticeably faster on the Synapse.

You mentioned that you tried the Ruby which is the Specalized equivalent of the Synapse. I didn't try the Ruby because although the Amira fits quite well, I preferred the more relaxed seat post angle of the Synapse 74.5 versus 75.5 in the Ruby in my size. The Synapse certainly fits a lot better than the Amira and for me better than the Ruby.

This is a long winded way of saying that for the kind of riding you say you want to do, I think you'd be better off with either the Synapse or the Ruby and you should cross the Amira off your list.