Just remember.... the diff if tight enough if it hurts your thumb! If you hear it squeak once on the track, stop and tighten it. Then go racing for a long time
The flex plate question: yes, there is just a little more fore and aft tension, but the t-bar does not make big movements that direction. I have not found that it's enough to spring up or down. Side to side is a little stiffer then a ball, but the lowering of the pivot point greatly out weighs that stiffness. Actually, it's better because of the additional rear grip you get, you will want the car a little flater to keep corner speed up. It's a win-win. The plate is roughly .016" thick made from a spring steel. Spacers will be sold with it that will bring the elevation of the t-bar back to where the ball was. The plate is sandwiched between two coned spacers to allow some rotation. A t-bar will never see more then a few degree of flexation ...
... unless you feel your way around the race track!