Is it possible to give too much praise in a reference letter that supports a green card application?
Letters written for a green card application are very differently structured to letters written for other purposes. As was explained to me when I went through the process, the structure of a green card letter is usually 


I am awesome
here are all the ways in which I am awesome
because I am so awesome, you should trust me when I say that this person should get a green card
and oh yeah, they're pretty awesome, which I can tell because I'm awesome. 


I'm only slightly exaggerating here. The point is that GC letters are not read by academics - they are read by lawyers who don't evaluate technical skills so much as achievements and strength of recommender. So there's no way to go over the top really.