I have done several sirloin tip roasts and I have one I am going to do this afternoon for us and friends for dinner this evening. I bought it from Costco, so, yes, it's big, but I will probably do half of it or so today and freeze the rest. I must admit I have only done the sirloin tip roasts in the oven, not in my smoker. It's been because of convenience more than anything. I slow cook them, however, in the oven. I put a generous amount of Kosher salt and a small amount of black pepper on the roast after I put EVOO on it. I generally start the roast at 250, then adjust, accordingly to how long it takes. I may bump it up to 275 or lower it to 230, depending on dinner time. I then let it rest for 15-20 minutes covered by foil. I pull it from the oven at 138 and it rises 3-5 degrees while it rests. My family likes it closer to medium, so your temperature can be adjusted to what you and yours like.
One thing, however, is that after it rests, I immediately slice it for dinner slices, which are thicker than for sammies. I slice it at probably a quarter-inch or so for the dinner slices. It comes out VERY nice and tasty. Yes, it's lean, but the slow cooking, I think, makes it very tender. It is juicy with plenty of natural au jus. It's MUCH better than top or bottom round. After we are done eating it for dinner, I refrigerate the rest and slice it very thin with my electric slicer the next day. GREAT sammies!!!