Scope zoom performance penalty on my computer has since Red Orchestra: Ostfront, always halved my framerate. Framerate was unacceptably bad, so I would almost never use scoped weapons. I have been away from RO2 for a few months. Don't know much about what has happened to the scope. I was able to compare the Battlefield 3 scope solution to how RO2 was like last I played it:
RO2 method
Renders a secondary camera view (portal-style) placed appropriately inside the rifle-mounted scope. Apply several lens optical/distortion effects to the 'scope' portal.
+High degree of realism and immersion.
-Very high resource usage. Usually 50% drop in performance.
-Scope view compared to reality may be slightly too bright, too large and too generous with eye-scope alignment (visible when bolting). But maybe the first two would cause other unrealism issues if true to life.
Battlefield 3 method
Lower player FOV (i.e. zoom in), while placing a probably manipulated in size and position 3D+2D model of the rifle scope in the center of the screen. If scope has magnification higher than x4, everything outside the scope's field of view is hidden by a solid black color.
+Very low resource usage. Highly optimized.
-Crude. Realism lacking. No special effects at all.
Summary: The BF3 methods (one camera, then sometimes blacken the non-scope vision) are not as pretty, but in-game it is an acceptable compromise, and vastly preferred, to the 50% drop in performance experienced in RO2. I often found myself wishing they would have an option in RO2 to just blacken anything outside the scope, if it kept performance up.