I would have to say that according to fluff, not the wargame rules, space marine bolters should be more powerful than sister bolters. It's frequently mentioned in the fluff that space marine bolters are much larger and powerful than those wielded by "lesser" humans
a space marine in space marine power amor is physically much stronger than a sister in sister power armor, it makes sense that space marine bolters are more powerful, but twice as powerful? probably not
