Hey there. I'm not an expert on hip hop vocals (or anything else for that matter), but I don't believe the pop filter, let alone the position of the pop filter, has much to do with how much sibilance (ssss) you capture. From my limited experience, the pop filter is there mainly to minimize plosives...hard consonants like "P" and "T" that can overload a condensor and cause unwanted clipping.
For sibilance, perhaps just try some slight reductive EQ in whatever frequency the annoying sssss are accentuated? I do know from reading other peoples' advice that there are de-esser plug-ins available, but I have never used any myself. The vocalist may want to consider modifying there delivery a little to minimize this at the source too...just some thoughts - best of luck!
