Photosynthesis increases until you hit the light saturation point (about 900 µmols in a non-CO2 environment) but, at this point, your plants are filling out rather than growing. My thinking is that the impact of turning down the PPFD a bit is that it's likely to be one of those "measurable but insignificant" issues. I did the same during the last few weeks of my last grow.
BTW, congrats on the grow. Your plants had all of the characteristics of well fed plants - very rapid growth, compact plants, and reduced internodal space.
Do the same for your next grow but ramp up faster - plants can be harmed by too much light but, if the plant can tolerate the light levels, they adapt to it "immediately". When questioned about the need to gradually increase light levels, Bugbee's replied that was unnecessary and said "they're ready to get to work as soon as they wake up".