as a fairly heavy rider too,
as a fairly heavy rider too, who’s broken more than one frame at the seat tube/top junction i’d tend to agree with shaun. by adding the shim you’re decreasing the area over which the forces are dissipated through that critical junction (unless you can find a really long shim). so it’s more likely to break.
you can get cheap carbon posts in 30.9 though… this one http://tinyurl.com/cqpgn8
is only 30 quid and you can get decent FSA and Kore ones for around 50 notes