The subject XDF matches up with 551S and 551AA and is nowhere near complete but has most of the commonly tuned parameters identified.  (I started using it on the 551S but found it matches up with 551AA as well.)  If you run into something that isn't correct, it can probably be found very close to where it is in the 551R, within six to ten bytes or so, sometimes double that, and generally, but not always, ahead of where it is in the 551R.  (I used the 551R XDF as the starting point and left all of the identified tables/scalars so that there is at least a place to start from.)

Nope.  Switch to the 4Z7907551R and the world will be your oyster, tuning-wise.  There are a couple of well-defined xdfs for the 551R on this site, such as last post here: 

http://nefariousmotorsports.com/forum/index.php?topic=2492.msg102934#msg102934