The benefits of Bt come into play when a field experiences pest pressure. Often these pest are corn rootworm and European corn borer. When an infestation of maize/corn pests is present in a field the Bt trait or traits in a plant protect a plant from damage immediately. In a non-Bt environment, a farmer won't make an insecticide application until he knows the threat is present at an economically damaging level. This application during the growing season will take time, equipment, fuel, and pesticide. This application may also kill non-pest or beneficial insects. Bt protection is there from the start when the seed is placed in the soil, and specifically targets pests that take a bite of a corn plant. So in years of pest stress, Bt plants should yield more than their conventional counterparts. But it is important to keep in mind that the presence of a biotech trait does not increase the intrinsic yield of a hybrid. A field with Bt corn planted but with no pest pressure may yield the same, more, or less than a conventional non-GMO hybrid. As of now there is no trait added to corn that just switches on a "more yield" gene. A good explanation of this can be found in a piece by Nicole Miller called "Value of modified corn is more in reducing losses than boosting yields."
Over a long period of time the protection Bt traits offer increase long term average yield. But in ideal conditions the presence of Bt does not increase the potential top end yield of a particular plant.