Decreasing the gain can reduce noise/distortion - however, that is not a problem with this design. It can also increase the risk of the amp oscillating. In case you don't know this, oscillation is NOT a good thing. Further, decreasing the feedback resistors means more of the signal current goes to driving the feedback resistors rather than driving the headphones, which is the whole point of the exercise.
In case you can't take the hint from spritzer, here's the bottom line. Unless you think the designer didn't know what the heck he was doing (in which case, why are you building his design?), I would ignore the suggested changes, and build the amp as designed. And this comes from someone who has done a fair amount of circuit modifications.