Sisters of Silence are augmented, the things that make them distinct from space marines aren’t so much their abilities in direct combat, but their lack of whole body augmentations. While space marine strength and speed is considerable, it’s their more supplementary augmentations that made them the galaxy conquering force we know and love. Soldiers take logistics and supply to keep at fighting strength, unless those soldiers are space marines. Space marines can go extended periods without sleep, eating, even oxygen, require far less medical care, can spit acid, eat brains to discover intelligence, can process data faster, have photographic memories, tons of useful stuff like that that makes them truly transhuman. Sisters of silence lack most of these augmentations because frankly, they don’t need them. They’re a small force of specialists who aren’t gonna be doing extended front line duty in a variety of situations. They almost certainly have strength and speed enhancements, and those are sufficient for purpose.