>>76588545 (OP)
>How much extra strength is it adding?
It reduces rom. The bigger the arch, the smaller the rom. That paired with all the other techniques, it's pretty much just reducing rom as much as possible to be able to move more weight. Lifting the ass also is attempting to get the legs, abs, and so on into the lift so that you can move more weight.
>Should I do it?
It's better to move through a range of motion deliberately depending on whether or not you feel the pecs. Especially if you don't care about powerlifting. 2pl8 with big pecs will come in time. 2pl8s with a big arch won't get you big pecs.
>arch
Now there is a difference between an arch and the natural shape of the spine around the spine's neutral position. The natural shape of the spine is S-shaped. So a portion of the lower back should naturally be above the bench. The difference between the arch and the natural shape is that most of the upper back is on the bench in the neutral position. Whereas the arch takes most of the back off the bench.
For pecs, it's best to stick to the neutral position. Not extending the spine in the arch and certainly not flexing the torso to try to make the back as flat as possible. For either BP or Incline, stopping an inch or two above your chest and pausing for a second will really wreck your pecs. I recommend doing it that way. Touching the chest often just leads to people either bouncing to use momentum to push the bar up or resting at the bottom taking tension off the pecs.