Interesting thing I just found out while researching Honda Care Warrenties.
The Pro-rated battery warrenty outline in the owner manual, is not modified by getting an extended warrenty. i.e. the same battery pro-rate will apply regardless of whether you have an extended warrenty or not.

While it seems as a general rule the batteries have been relaible, these are "relatively new", and therefore why I considered the whole extended warrenty in the first place.