You can calculate the stock bar stiffness! I don't care, so I'm not going to do it, but that's something someone who's as serious as you guys about your suspension might want to consider doing. Then you have a number to compare to! You can even calculate your progress bar, and then take all that into account when you design your new bar!

If you're going to go through all the hassle of custom fabricating parts, you might as well do some engineering calculations to support them. It's going to take you very little time on the overall project, and the end result will probably be better than if you just eyeball it...

And the bar calculator is assuming its going on a race/high performance car, so I'm sure it just assumes zero deflection. Poly bushing or spherical bushings should both have zero deflection.