I think it does have to be at least 155, any time I've tried a narrower saddle I've felt as if my sit bones were falling sideways off the saddle at the back.
By flat, I meant not dipped in at the center (the SMP saddle is an example of that). The channel in the Arione isn't very deep, it's just about right, and the saddle is quite flat.
Unfortunately, no shop here sells Selle San Marco. I'd like to be able to send it back if it doesn't work, and that's expensive when buying online.