I've measured the ID of the shim and OD of post and there's a 0.1mm difference. I don't know if that's considered standard, but it seems functionally to be slightly too large. Something like 0.06 would be better.

I'm going to make my own seat post extender in the next few weeks so I can use a standard suspension seat post. I reckon it'll be cheaper to make than to buy the telescoping seat post, and besides, nowhere stocks it in the UK. Should I decide to stick with the standard seat post I may try adding a bit of thin tape around the outside of the shim to tighten the fit, though getting it out in the first place looks tricky.