Andrew R Stewart 

My usual first way to open up a squeezed in seat tube, at the binder, is to use the seat post as a lever. The frame will have some spring back so plan of levering it open further than the final size.

I have to say that looking at the binder slot and how parallel the edges of it are I wonder about your speculation of "correct" post size. It's not uncommon for a seat lug to suffer from heat induced deformation and a decreasing of the fore/aft fit. A factory could ream the seat tube, and risk loosing some tube wall just below the lug. Or they (or the LBS that assembled the bike) could select a slightly smaller post and if the binder held it tight enough to stay put all would be well.

So My first question would be does the seat post stay put or does it slip down under use? Many here have found it good advice to not try to fix a working part... I also wonder if the plumber's putty was to try to seal the post from water getting down in the frame. Have you pulled the BB and checked the condition at the base of the seat tube and that of the shell? If you found rust my speculation might hole water (bad sort of pun) Andy