Not sure about carbon posts (which I don't care for) but, if done properly, I can't see any problem with using a shim. I have a Basso Viper that takes a somewhat unusual 30.0 mm post. A nice 29.6 mm ITM Millenium post came my way (I'm using Millenium bars and stem) so I bought some 0.008" (approx. 0.2mm) thick brass shim stock from a hobby store. Works fine. You can find shim stock in a wide variety of thicknesses if you look around.

Thomson makes a nice 30.0 mm post and if I get a good deal on one I might switch, but I'm not in a hurry.