Hi,
my wife and Sis have Thompson seat posts;they are light and nicely made. My Sis has a bike that's a little large for her; and the seatpost places her knee right where it should be. My wife said she felt a little cramped on the bike; so I put her old one back on.
If you can use the Thompson Setback, that would likely keep the knee over the pedal. If you're happy with the fit of the bike from the waist down; the regular Thompson is likely to bring you too far forward.
I mention t all this because you have the seat back already. You might want to look at posts that allow you to move the seat back more; like the Raceface XYZ. There is a post that comes on (I think) Masi bikes that is light, cheap, and has almost as much setback as the XYZ.