You can’t just Mulitply these together. Multiplying anti aliased passes together will always cause this sort of problem. A lot of times this can be corrected by dividing the “reflection” buffer by the “raw reflection” buffer to create a new (but slightly different) reflection filter buffer. Then you can make alterations to the raw reflections and mult that by your new reflection filter buffer.
This sort of math is really best handled by compositing software like Nuke and always done in floating point. In fact I don’t think its possible to do this in Photoshop. (someone correct me if I’m wrong).
Here is an example of a similar problem but with raw light passes and the diffuse buffer getting multiplied together. Your problem is the same thing but with reflections. This problem can be fixed using the dividing method described above.
Tim J