No problem going with a 28cm extension given your needed adjustment range. I regularly use my 28cm Chinook RDM extensions (I have both carbon and aluminum versions) at 28cm with no problem. Earlier, when I was using comparable Fiberspar carbon SDM extensions, again no problems. The pertinent point to remember is that extensions are made to use at the highest point of extension, yet they also include the extra minimum length needed that inserts into the mast to ensure a proper bearing surface.
As an aside, the carbon content of the masts you noted are quite low, and as a result, they will be heavy and not offer very active responsive response, especially when using larger sails like the 8.5. Also, you lose a notable degree of the sails' designed range, particularly at the high end. Really, you might want reconsider going with 75% carbon masts and gain a significant performance improvement, but still maintain great durability. More expensive? Certainly, but in my opinion, worth every penny.
Last edited by steveC; 4th February 2010 at 05:28 AM.