Arm peak can be dramatically improved regardless of genetics. Larry Scott, in his book loaded guns, shows pics of himself in the 1960's. he had great full biceps, but no great peak. Then, in the late 80's, early 90's, he trained using a lot of peaking and isolation movements. He shows comparison shots and his bicep peak is way improved. Even Sergio, in the mid 80's got a better peak on his biceps.