I highly doubt you could fit everything the Borg knows into a humanoid brain. Though I think a general catalogue of species and the capabilities is probably a good thing to keep in the local cache. As for Seven's other knowledge, she probably got whatever was in her organic brain and stored in her cortical implants when she was disconnected, which may have just been bits of Borg knowledge useful to her current assignment, ship, mission, etc..
She certainly didn't know everything the Borg knew - though - considering she didn't know that her parents had survived, despite apparently both operating close to the Queen while drones.
Most of what Seven or other Borg disconnected from the Collective know is likely stored in their implants.
Remember that when Seven experienced issues with emerging personalities the Borg assimilated due to a faulty vinculuum, it was stated that those personalities resided in her implants.
Its likely the knowledge of the Borg she retains and her eidetic memory are attributed to Borg augmentations.... and besides, Seven herself stated that only useful information is retained from the knowledge... and that in itself makes her one of the most intelligent human beings alive.
Think of it like this... with say Brain User Interfaces, or far superior/better/safer artificial neural networks - which are also technically possible to create with a bit more research using nanoparticles and programmable metamaterials, you would not only have access to all the sum of human knowledge via say wireless connection, but also a possible way to upload your consciousness to a computer (not create a neural copy though) - though transfer of counscisncess would be more likely with ANN (which would also be infinitely safer) rather than a 'simple' BCI.
An artificial neural network for example if grafted onto the biological one and could greatly enhance what you remember, the speed at which you access it, etc. (and it can be used to aid in treating/curing neurological disorders). We already made synthetic neurons and other things that can be used for creation of ANN.
In Seven's case, the knowledge she has is either split between her brain and her implants because it was integrated into her from when she was assimilated and in the marutation chamber... or majority of it is in her brain, with the implants acting as a bio enhancer (and an equivalent of an artificial neural network) and extra storage space for storing excess information (such as assimilated personalities).
So, even if Seven's cortical implants were to be removed, she would still likely retain most if not all of her Borg knowledge, but the excess personalities and data stored in the implants themselves would be gone - we also don't know how removal of these implants would affect her memory (did she always have eidetic memory, or was it one of the enhancements she got as a Borg - and did her human brain adapt to this change permanently)?
