Haven't done top sirloin but I have done bottom several times. I agree with the more modest cooking time. In fact, I do it a bit less, eight hours. If you go long, it'll taste like pot roast. Even at the moderate time, it'll taste a bit pot roast-ish, but not nearly so much. Meanwhile, I don't think it's tender enough cooked just to temp. Of course, YMMV.
ETA: To clarify, I also didn't think the long cooked bottom sirloin (what we here on the Left Coast call tri-tip) made a very good pot roast. The texture was too tight for my taste. For pot roast, I prefer chuck. Again, YMMV.
