Ok, thank you for filling me in on the details of this issue. Yes myself and another user didn't run into the limit because we are both in the same situation of having 4 XBox360 controllers, and then some generic USB gamepads that were kicking around to make up the other 4 controllers.
It seems like this will affect only a percentage of a percentage of users, so I'm not sure how highly I can rate this, especially since it would likely require me cracking open Godot and re-doing how they handle input, OR re-writing it from scratch in SDL etc . and handling the input myself... neither of those options are trivial and would be entire projects in their own right! (The funding just isn't there at the moment!) That's just my current assessment though, and I'll be keeping it in mind and looking out for other complaints. The forum will be up soon where people can post about any issues, and who knows, if the paid version with extra features take off, it could warrant further development in the XInput handling area!