Requests for comment/Business Layer Architecture on budget

Overview
I would like to propose a simple and inexpensive way to migrate to Business Layer Architecture that would greatly simplify our codebase and allow very rapid UI improvements. It involves a relatively easy refactoring of the current API subsystem, followed by a gradual "as needed" migration of the rest of the code.

My assumptions

 * making SQL queries everywhere is bad. There should be dedicated layer that knows table structure, checks permissions, and performs all db operations.
 * Allowing all PHP code direct db access can cause security and data integrity issues, and results in poorly tested and tightly coupled dependency hell.
 * UI rendering PHP code should have access to the same capabilities as JavaScript, allowing us to quickly change UI and support both legacy and modern browsers
 * usually it is better to reuse well tested code than to write from scratch

Problem
Our API already implements most of the business logic functionality. It does optimized SQL, permission validation, batch processing, query continuation, localized error messages. From the functionality perspective, API classes do everything that BL logic needs to do. The usefulness of the API even outweighs the difficulty of using it internally, and we have plenty of ApiMain calls in PHP.

But, API tightly couples with the output formatting, and it mostly operates with strings, not objects. While 90% of the API module code is BL functionality, the remaining 10% is formatting.

Proposal
My idea is to decouple formatting from the API, and let API modules work with objects such as Title, TitleValue, User, etc, instead of strings, for both input and output. The thin wrapper on top of the API would de/serialize objects as needed, and expose it to JavaScript and bots. Internally, the code should also access database via API modules, except that they would use objects, and have better interface than FauxRequest.

Concerns

 * Performance : While obviously a concern, the actual API usage does not add significant performance cost. Passing objects as parameters would avoid any marshalling costs. Additional parameter validation should have minimal impact and would ensure consistent security model.