If your goal is to save weight, the flatback is probably the heaviest light thing you can do. Yeah you save weight over the powertrol, but leaving a lot of unneccessary iron on there.
I had a flatback on mine. Removed the flatback, seat pedestal, hitch and adjusters off and made a "all-in-one" cover, hitch, seat, adjuster and saved about 80# over what I had.
The channel seat mount was about 50#
The hitch, drawbar, etc, was 75# (don"t forget to take that triangle piece off under the tractor).
The flatback I removed was about 10-15# if I recall.
This was on a B, on a G, all those weights would go up and the all in one would weigh 60# give or take.
The new combo weighed 46#.