Man, I'm blind. Even after Rich said you said 250 I still didn't see it right away.
There really isn't much need to take grate temps as well as lid--one o4 the other is fine. If you are running a conventional set-up (with water in the pan) your temps at the top grate should have been 225-235-8, the high side of that more likely. (Of course, check your lid therm for accuracy. What are you using for a lid therm?)
While it's tempting to say 'all butts are different' or some such that really isn't the case. There is little more standardized than production pork and you're right, 16+ plus foil is not what one would expect for 5.5-pounders unless you were running at temps 20 degrees (or so) below what you thought you were. It would seem that either the therm is off or the placement of it skewed the number.
(No, btw, you don't want to put a therm on the grate. You can run it through a potato and put that on the grate but then you still have to open the lid. However, if you want to try that, you can position the therm face in such a way the you only need to open the lid very little--just enough to see the therm, lifting it straight up to retain heat--then replacing it right away.)