I heard that guardians' fps are more sensitive to variations in DL and DW than other bows, is this true?
I'm not sure where anyone would get that idea? Any bow is going to lose a considerable amount of speed dropping the draw weight down 10# while keeping the arrow weight the same.
Every # you drop the draw weight is like adding 5grs to your arrow's weight so dropping 10# is the equivalent of adding 50grs of weight to your arrow which will drop your speed probably 15-18fps?
Some bows are a little more efficient at certain draw lengths and all bows that I have seen are most efficient at their peak draw weight but I wouldn't have any reason to believe that the Guardian would be any different than any other bow in it's IBO speed range by adjusting the weight downward.