Regarding circle leader's maximum spell level, but not Heighten's max spell level: Fair enough. The max spell level, even including epic, is 9th then (with Extend and Empower having no effect on spell level, but spell only slot), as the ability specifically fails to mention any feats other than Empower, Extend and Heighten. That's too bad; it limits the ability to make spells nigh irresistible.