Is this true :
"Turn off collisions on objects that don't need them"
No, it's not true. As of r161 objects which don't have any collisions tested against them do no collision-related work at all, so the option has no effect if you already don't test for collisions on that object.
Even before then it would have made such a tiny difference as to be unlikely to be measurable. As with any performance question, measurement is king: if you cannot measure a performance difference, it either does not affect performance, or has such a small effect as to be ignorable.
Everything likely to affect performance has already been listed in Performance tips. Anything not in that page is not generally something that has a significant effect.