Not a lot of difference. If someone insisted that I assign numerical values "nothing more" would be 0% more, and "little more" would be 0.1% more (or something like that). I don't think that the phrase "a speck in the rearview mirror" is often used in daily conversation. Maybe because it's not a concept that we need to express often in daily conversation? But it's common enough that most people will understand the meaning. Google it and you'll find lots of modern examples of it being used.