Hi Nola! I haven't chimed in on this one yet, because I'm not sure what to say. I'm not familiar with a bone-in sirloin pork roast...is that what was on the label? I'm just trying to figure-out what cut that is. This is important because it will make a difference, depending on what results you're looking for, i.e., sliced or pulled pork.
At any rate, brining is always a good idea with pork! You won't need to let it swim in the pool very long, maybe 4-5 hours. 3.5 lbs is pretty small, for a pork roast of any kind. If you plan to slice it, take it to no more than 175 internal temp. For pulled pork, 190-195.
For your next "big hunk," I recommend something like a bone-in Boston butt (pork shoulder), greater than 7 lbs. The reason I say this is that those little cuts cook unpredictably. Pork isn't as bad as beef, about that, but they can fool you.
Sorry I can't be of more help, but this one was a bit of a puzzler for me!