A marine without power armour is dead in the water against a competent foe.
Not really, even without the armour they are still modified transhumans that can think and react faster, are tougher and stronger, and have additional organs and redundancies to keep going on. Power armour enhances a vast majority of what already make them superior weapons, it doesn't define them wholesale.
Would an unarmoured marine be able to tank bolt rounds? No he would not, but a marine in power armour wouldn't try to do that either.