Seatpost worries - Help required

There was a 27.2mm seatpost in my 753 frame when I got it.
I carried on using a 27.2 - but longer.. this is what happened:

http://www.lfgss.com/post242277-1.html

Further investigation by using different sized posts (thanks retro di corsa) suggested the seattube was meant for a 27.0.

What was your bike like to ride ? I've never riden my 753; is there much difference compared to 531 ?