Even if there were an overhead, I'd rather have the stability of a fully tested framework than relying on an implementation with no tests.

What you have essentially done is created a framework, which in order for me to extend your CMS, I am going to have to learn. I also would lose the added benefit of not being able to use existing packages for (insert widely used fw here).

I can't really understand why you aren't using composer as well, it seems as though you are managing your own dependencies in the vendor/ folder?

Look, I have respect for you going out and creating your own cms. But I seem to get constantly disappointed when I dig into the nuts and bolts, and unfortunately this one is no different.