I recently had the Thorn Clarke Wiliam Randell Shiraz from the 99, 02, and 04 vintages. I saw no benefit from the additional age.
The only reason I would buy a shiraz is if I was looking for a mouthcoating, tooth staining experience and that is what there was less of with the 99. It seemed to have more finesse and grace but again that's not what I would want from a shiraz.
I am not however saying that shiraz isn't capable of aging just that I personally prefer them young.