Sorry, I've only been using AP cages for the past few months. I haven't noticed any sagging, but that doesn't say much.

With AP cages you can either get swinging or sliding glass(/plexiglas) doors. Either way, the cages come with a thing separating the sliding doors (if that's what you choose) and the inside of the cage. It's difficult to explain.

Here's a pic of my AP T10 (sliding glass doors).



So you could probably pile 3" or more of substrate without any getting in the sliding door tracks.