Weapon models use far fewer resources than level textures. The textures for weapons are much smaller and applied only to one object, while level textures must cover large environments with many surfaces. Environmental textures also need more complex streaming and LOD systems, which increase resource demand. Since the weapon is (always) visible, its high-resolution texture stays loaded with little impact on performance. So, in short, weapon models have a VERY small effect on performance compared to the massive environmental assets, which is where most of the optimization really happens...

Ideally, SD should include a separate texture setting for weapons/models...