The typically failure mode for the seat post is to bend right where it comes out of the frame, usually it will not break, but just bend. I did have one bend, and then I ran a stress calculation (I am a Mechanical Design Engineer), and found that the seat post bent at very close the expected load. The bending stress that a seat post sees is proportional to the distance from where the seat post enters the frame to the top of your saddle. The stress proportional to one over the diameter of the seat post raised to the fourth power. Thus a 27.2mm seat post is more than 25% stronger than a 25.4mm seat post of the same material. The inner diameter has very little effect on the strength of a seat post. That all said, for us 350lbers; I would suggest 8 inches or less from seat post exit to top saddle with a 25.4mm seat post, 10 inches or less with a 27.2mm seat post.