I don't think there is a tenable middle ground. Mages are demonstrably übermenschen.
Not so much. If I remember correctly, mages have different level of ability and power. Powerful mages could be seen as übermenschen, but the same applies to the highly skilled Templars and their powers. Mages with weak magical abilities, on the other hand, are not much different from the mundaines, with the exception that they're more desirable to demons, and possession would/could turn them into übermensch, but, as we've seen, this is the case with mundanes as well - damn, its even the case with trees, since sylvans are trees possessed by spirits. So at most we could say that mages have a higher chance of being/becoming übermenschen.
Plus, if the mages were truly absolute übermenschen I doubt they would've stayed under the Chantry's rules for this long.
Also, I apologize beforehand, but I just have to: we're using the term "übermensch" incorrectly here. To Nietszche, the term was more existential and intellectual than biological or physical in nature: he used it to refer to the people who'd accepted Zarathustra's phrase "God is dead" and who had abandonned the traditional notion of humanity and judeo-christian morality. It's more about going against predescribed societal "truths" (particularly organized religion and its dogma) than about being physically superior, which is how the term is usually (mis)used. If anything, the concept of übermensch is about self-realization, perseverance and independent tought. So if anything, the Circle mages in particular are an antithesis of übermenschen since they stick to a (religious) dogma and authority (and a set of societal rules) and don't think for themselves or try to exted their abilities or thoughts beyond "what is allowed" - so technically they're Nietszche's opposite of übermenschen - "the last men".
Again, I'm sorry. The misuse of this particular term is like my Number One philosophical pet peeve of all times.