Mondo is one of the most accurate ways to measure a boot. I measure at a 27.5 on the left and just shy of a 29 on my right. I can get into a 28 with minimal work. The problem lies in the foot model that was used for the boot. Each company uses a different last and foot model. With an AAA width you're definitely going to end up in something like a Northwave, certain Salomons, or maybe a Ride. That's ungodly narrow so getting a snug fit will definitely be an issue.
The thing to understand is that some companies measure the liner size and others measure the sole size.