The 338-416 imp is about the most you can get out of a hunting weight rifle. The wildcats off the 408 chey tac case will get you 150 more fps but you are looking at a 16+ pound rifle that costs a fortune to shoot and load for. 416 brass is fairly cheap. With the choices you listed I would do the 338-416 imp custom and I think you will be very satisfied. The only thing that will outshoot it would be something off the chey-tac case or the 50 bmg case. If it is for hunting I have big 338's off the ultramag case, lapua case, 378 wby case and the 416 case. They vary about 300 fps from the slowest to the fastest and I have taken large game with all of them. They all kill very well with about the only difference being the faster ones take a little of the trajectory/wind variable out. Time of flight to target is important.