How to cook a pork sirloin roast?
I don't think a pork sirloin roast will stand up to that kind of cooking very well.  That is a very lean piece of meat, which does not lend itself to long cooking times at low temperatures.  That type of cooking is best reserved for cuts of meat with lots of fat and/or connective tissue.  If you put a pork sirloin roast in your cooker for 8 hours on low, you're going to have one tough, dry piece of meat when you are through.

You would be better served by cooking it in the oven at higher heat, for a relatively short period of time, like 400F for an hour or so.  Bear in mind, you're not going to get to a fall-apart tender state with a sirloin roast.  This isn't a pork butt with which you're dealing, which you'd take up to an internal temperature of 195F or more.  Rather, you're going to want to target the safe temperature, and no higher.  You want the meat to be juicy.  Cut into thinner slices to achieve maximum tenderness.