What a coincidence, I just got Salomon Synapse Wides. I use them in large Flow Pro C-FR bindings, which are pretty nice. This combo on the LSD begs to be carved, even in the super-sloppy boots I was using before the Synapse.
A couple things about the Flow: they are not as 'step-in' as they seem. Sometimes they are easy to get in/out of and sometimes they aren't. I've had a lot of trouble with them lifting the heel when closing the binding. On a recent wet powder (Sierra Cement) day, various pieces packed up with snow/ice and wouldn't let me adjust the tongue or close the lever.
I'm not so sure you'd want to use Synapse Wides, Flows, or any soft setup on a Prior 4WD or Coiler AM, which are too narrow for softies at low angles. They are hardboot boards.