Sirloin is a better cut beef, so you should receive excellent results as well. Cook's Illustrated considers Sirloin Top Butt/Tip Roast to be the poor man's Prime Rib. Slow roasting is how companies like Boar's Head and Thumann's prepare their deli meats ....usually for 8-10 hours @215-225*.o roast beef slowly, preheat the oven to 250+ degrees F. After it is preheated, wait an additional 20 minutes before putting the roast in. Then put the roast in the oven and/or lower the temp to 200-225 F. Cook the meat until it gets to 120 degrees F (2.5–5 hours).