Can I lie about my GRE score to get a better letter of recommendation?
I am tempted to say: "Never ever lie on an issue like that." Actually, I say: Never ever lie on an issue like that. First of all, it's wrong, plain and simple. Second, from a more practical point of view, you never know how it's going to bite you back. 

It can taint you forever, even if here you are downplaying your achievement. It will be difficult for people to believe that you actually did that, and they will start believing that you are untruthful in other aspects, too. Frankly, it will raise more than a few question marks if it should come out. 

You'll have to live with the exaggerating letters, or else to find someone else to write you a sensible one.