Pre-bend dosen't increase the load capacity of the beam/dolphin striker assembly. On the contrary, it decreases it somewhat.

To achieve the pre-bend you have to put the dolphin striker under tension. This puts the main beam under some compression and some bending. The rig load also puts the dolphin striker under tension, and the main beam under some compression and some bending (the applied bending moment is opposite to the pre-bend induced moment, though).

The net result is that the pre-bent structure has higher tensile stress in the dolphin striker and higher compressive stress in the main beam, although the tensile stress will be reduced or eliminated in the main beam. Assuming you were starting out with an efficient design, both the dolphin striker and the main beam will have to be made stouter to take the pre-bend.

Pre-stressing is a neat trick for concrete structures, because concrete has very poor (generally assumed to be zero) tensile strength. There's no point in pre-stressing a steel or aluminum structure except for deflection control.