07 350mpi won't go at wot

i'm new here looking for some help that will be greatly appreciated. motor is in a gekko ski boat. only go up to 3/4 throttle engine runs fine. give it anymore feels like someone threw an anchor out. motor sounds flat, doesn't bog but seems like a car with a dirty air cleaner. changed plugs, checked flame arrestor, water/fuel separator, and fuel pressure to see if it dropped off. it was at 40 psi, mechanic checking it said it seemed low cuz all other friends boats are at 60. my brother is a gm mechanic and he said it seemed low. is there anything else that I can check? to me if it was injectors it would run bad all the time. any help would be great. thanks, brian