Sirloin Tip is one of my favorite cuts for making an oven roast. I like to salt and pepper it heavily all over and then slather it in olive oil, garlic, and herbs. If you can do it the day before, even better.
Let it sit out at room temperature for an hour or two before roasting so it can warm up a bit. Then put it into the oven at 225-250F and cook it to an internal temp of 120-125F. Take it out of the oven and crank the heat up to 500F. Let the meat rest while the oven comes up to temp. Then return the roast to the oven for about 10 minutes, or just enough to brown the outside.
This roast really benefits from a nice long rest. Tent it with foil and walk away for about 30 minutes. It will be juicy and flavorful.
The leftovers also make wonderful roast beef sandwiches.