At the time he fought Minato, he was certainly upper kage level. Minato himself was easily considered an upper to high kage and Obito proved he was a proper match for him without doubt.
To be honest, mid Kage level is not all that impressive. We all know that the Leaf is the strongest village but their best candidates for Hokage, aside from Jiraiya, were Tsunade and Kakashi. That alone should tell you many kage in the lower to mid level are not all that impressive. Not to mention, weaker villages like the Sand and Mist have virtually no kage candidates other than their standing Kage, this also proves the weakness of an average kage.
Obito easily wipes the floor with most of these kage and kage-candidates, making him upper to high kage without doubt, even at that young age. There are other pieces of evidence like how easily he dealt with seasoned anbu members, which automatically puts you in the kage conversation, but his abilities alone are enough to prove it.
kakashi was a low to average kage, its mostly his intellect that gave him more potential and promise than his actual abilities deserved. Without MS, Kakashi is weaker than someone on Darui's level, but his intellect boosts him.