First I want to clear up a little misinformation here.
The Rambox does not "really limit your bed size" as much as people think, and it relocates some of the less usable (for me) space in front of and behind the wheel wells to the area inside the bed sides. I actually like the flat walls for the things I usually carry like boxes. And unless you're loading light material, fully loading the bed with something that will utilize the awkward spaces in front of and behind the wheel wells (firewood, gravel, dirt, etc.) you will exceed the payload limit anyway.
The Rambox option is more than just storage in the bed sides. It also comes with a bed divider (if you also get the BUG). The bed divider is at least as useful as the storage boxes, if not more useful. There are multiple mounting points to custom fit whatever you're carrying. And it also doubles as a bed extender if you add the mounting pegs (about $50).
My opinion is that the majority of people who say they don't want Ramboxes because they will lose bed space are those who have never had them or used them. I would know, I used to be one of those people.
The MFT is more than nice. It's a game changer for me. I'm getting up there in age, and my back isn't what it used to be. And I'm sure I don't have to tell anyone how much of a pain it is to drag cargo over the tailgate when loading or unloading it. With the MFT there is no stretching over the tailgate, and my back doesn't complain as much when I load or unload whatever I need to carry. Finally, the MFT does NOT require any extra bracing with the Rambox, which is what we're talking about in this thread.
You're right about the Rambox and MFT affecting payload. It's definitely a consideration, so if you are looking for max payload I'd probably avoid them both.
In addition to the Rambox and MFT I also have the Advanced Safety Group. I like the ACC but don't use any of the rest of it. I don't use my truck to tow anything, and rarely carry much more than a couple hundred pounds in the bed. So for my use, If I had to make a choice it would definitely be the Rambox and MFT.