We just reduced from 7 to 5 knots additive. Did a couple of autolands on the 300 last week, one in real 200 metre RVR, and the 2 knots did not seem to make a difference.
I have heard a theory that additional speed might make for a smoother touchdown . But compared to the 700 the 300 is generally better in that area, at least from a passenger comfort perspective.