Custom Authorization based on the business logic

I will be working on redesigning the big SharePoint document library with authorization rule based on the business logic that is coming from the different sources like the document Meta data, Database tables and other application through web services. The library has about 100K documents and the site performance is crucial for the business. The Item level permission could not be a solution due to the performance issue. I am thinking of making the document library secure and writing the custom form for uploading the document and for displaying the documents based on the user permission as per the above sources using the elevated account. I want to check if this is a good way to approach this requirement.