I've used the XDP Power Plus stuff in my OBS, couldn't tell you how much difference it actually makes, did a lot of other mods to the truck around the same time I started using it. I haven't put any in the 550 yet, the bottle was still in the OBS when I filled it up, so the next time I fill it up I will add it and see if I notice any difference.
The main reason I decided to to try something was my truck doesn't get driven a whole lot, so I figure anything to work as a fuel stabilizer would be helpful.