Without going and measuring, I reckon the min insert of a Thomson post on the mtb is around 11/12 cm - thats on a 44cm post. I'd say you could quite safely cut it to have an insert of 15cm (probably a bit less).
Hi OK 10cm it is. I want to cut it because it takes a ridiculous amount of time to get in and out of a frame and it's tricky looking after a 40cm greasy pole (oo-er) when you remove it for security. As it has my Brooks attached to it, weight isn't exactly the issue!