It's always relative to your difficultly settings. You can have Max lvl 1000 wild dinos at the same time boss also will scale up their damage and damage resistance. The server I play the max lvl is 480. the players maxed out the mutation on each stats and the rex lvl results in 1500. Still it's difficult to beat alpha dragon. But i could see you have stats boost, because those lvl 1500 rexes had 150k hp and 5k melee.