Depending on how your scene is painted, you might use a set of low contrast materials for rendering the reflection pass. Before rendering the cube map, paint everything with it's low contrast material and then afterward paint it back with the normal materials. Exactly what the materials should be depends on the blend mode you use, but for example, with add blend mode, a completely black material gives you nothing at all, so if you worked in dark to medium shades of your colors, it would temper the affect and save you having to post process anything.
Since your cubemap res is so low, I'm not sure if the post-processing cost would be higher or lower. Just a thought, based on something similar I do with per-object glow effects in my own game engine.
Since your cubemap res is so low, I'm not sure if the post-processing cost would be higher or lower. Just a thought, based on something similar I do with per-object glow effects in my own game engine.