Regarding the seat post, you should be prepared for a bit more shock coming through the larger diameter post. Maybe the carbon will offset it. The 27.2 post with shim is a bit more flexible, construction material being equal. I had always ridden 27.2 mm posts for nearly 30 years before building up an Everti Falcon Ti frame with a 31.6 mm post. Same carbon post model as on my other bikes, but the larger diameter was quite noticeable as a very rigid feeling under my butt. Three years later I am fine with it. Think about it before you choose since you already own the shim.