If you need to filter by extended fields, I think getCollectionGraph() is probably necessary, otherwise, a simple getCollection() call would do it.
It might be possible to get the count directly from the extended fields table with either getCount() or getCollection(). That would be faster, but I think you'd need a new $where to do that, and it probably shouldn't be added as an official part of the extra.
I'm glad I could help. Thanks for reporting back.