This use of of is explained in Collins Cobuild English Usage (p465):
You can sometimes use of and a noun group to describe something,
instead of using an adjective and a submodifier. For example, instead
of saying that something is 'very interesting', you can say that it is
of great interest. This is a rather formal use.
- It will be of great interest to you.
- The result is of little importance.
- ...a film of considerable character and intelligence.
You could restate Kant's sentence as:
Far more important than all that has been above said is the
consideration that certain of our cognitions rise completely above the
sphere of all possible experience.
But as the usage guide states, the noun phrase is a little more formal and thus better suited to such a philosophical text.