By making small mesh adjustments, I haven't had a Firm Feel rebuilt box that is any different from the Borgeson units. Any slop in the steering is induced by deflection in tires, components or alignments that cause steering center to be off.
FWIW, we have cars with both boxes, we sell neither, so we are about as unbiased as you can get. I just want you guys to have fast cars. If the Firm Feel guys say your box is toast, or you are converting to power form manual, then I would say that Bergman or Borgeson is the way to go. If you car is already a power car, save some coin and rebuilt and adjust what you have. And remember, the mesh is a fine tuning to a larger adjustment.