These experiences of my own may help.
In 1981 I had a 27.00mm seatpin - it was Campag Record that one of my LBSs had surplus and couldn't sell, so I'd not been able to resist a Campag for a bit less than half price. I inserted a strip of takeaway alu tray between the seatpin and the inside of the tube. It was 531 tubing and 27.2mm. I did have to wedge the seat-lug open a bit to get the ensemble into the tube, but it stayed there in my frame for over 20 years. All was well.
On another frame, about three years ago, I couldn't get my Brooks saddle far enough back, but replacement with a Spa saddle solved the problem. If your saddle is a Brooks you may be aware that Brooks saddles - comfy as they are - are notorious for not having sufficient fore-and-aft adjustment.
I have had a Velo-Orange 'Gran Cru' seatpin for a few years and it solved just this problem for me on a frame where the seat-tube was too steep.
However, there has been a thread on this Forum questioning whether the current and redesigned VO seatpin is going to be reliable. It seems that there is another seatpin, which appears as though it could be identical with the original VO 'Gran Cru' but under a different brand name........it may be worth looking into that before spending the money.
You may find it if you click on this :http://www.planetx.co.uk/i/q/SPHOGS/hol ... t-seatpost