[Zope] BTree Folders questions

How many objects should one have in a single folder to consider using BTree Folder over the regular Folder? Does it make sense to use a BTree folder for 100 - 500 objects or should it be reserved for situations where there are say 1000 + objects or more. Are there any benefits to be gained at the lower end as well or are there drawbacks? On another note, why do CMFBTree folders not appear in CMF? Is this the way they are supposed to work? If so, what differentiates them from a BTreeFolder2 if they are not a type visible to CMF? Many thanks. Regards, David