CDR, I am also about 5' 7" (when I don't slouch) but we're at polar opposites when it comes to bike fit. My legs and arms are long-ish for my height. My femur is proportionally very long for my leg length so that causes me issues with respect to being able to get far enough behind the BB spindle. Even if I could get my back as flat as yours while riding, I wouldn't be able to see because I have a short neck and I can't tip my head up as far as most people even though my neck is relatively flexible.
On top of everything else, old age and weight conspire to make me ride in a higher position. To combat that, I am constantly working on getting as low as possible, as much as possible. Still not as low as when I was younger.
