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.
A normal domestic oven never heats strong enough to produce a crisp crust. This is why pan searing is usually done: the direct heat transfer causes the maillard reaction which is impossible in the oven. As no setting in the oven will give you that, you are better off using lower temperature. It takes longer, but makes for a more even heating, which results in a better roast with a smaller heat-gradient-overdried part.
If you want your browned surface, you will have to give it a good heat blast before/during/after the cooking. I agree that a pan sear will probably ruin the seasoning. If your oven has a grill, you can use it. It shouldn't matter if you start with the grilling and then roast to the end after the skin is good, or first roast it almoast ready and then grill it until the crust is OK. Any way, counting the time spent under heat will probably not be acurate enough with this technique. You can try it, but the error probability without an oven thermometer is quite high.
The other possibility are open flames. A kitchen butan burner seems to be the tool of choice for sous-videers, who of course never get a crust in the cooker. But it will probably char the spices even more than the pan searing method. You could get creative and finish the outside of the meat on a charcoal grill. The reason nobody makes thick steaks and roasts on it is that they don't cook through. With a preroasted meat, this won't be a problem.
If all this sounds like too much trouble to go through for a simple roast, you can just roast it slowly and eat crustless. Just because a crust is traditional, it doesn't mean it has to be there every time.
Best Answer
Preparation is key, as with most meals. The difference (in tenderness) between a pork sirloin and a pork tenderloin is relatively minor but the tenderloin is generally more tender (as the name implies). In this diagram you can see that they are adjoining cuts from the same primal cut (the Loin). Once you start preparing these cuts the final dish may result in sirloin that has been rendered more tender than the tenderloin. A slow-roasted sirloin may end up 'more tender' than braised tenderloin.