If someone could make use of that bespoke cf K member, the non stock chassis would probably benefit from altered locating geometry, negating the benefit of a bespoke K member. So then alter the CF K?
I would also posit that if one optimized the chassis and unibody, the stiffness of the k member would have a diminishing effect on chassis stiffness so that would reduce the loads on the CF K and it could be made less robust and lighter? and that a decent motor plate would take care of most of the remaining forces needing resolved up front, which will tend to be mostly lateral. Not sure any motor plates add any real usable stiffness
One of the biggest chunks of mass may not have a huge moment arm to the K member, but plenty of the rest of the car does, including a hefty fuel tank and rear end. The car rotates on the rear axle in full grip situations The rear end that is also putting all the motive force into the chassis. which propels the car nearly always forward in a single direction and maybe under acceleration reduces front tractive reaction forces and under barking reduces rear tractive reaction forces A chassis which also comprises 1/2-2/3 of the total weight, with its own cg pretty far aft of the front wheels and which has to take all the braking and rolling forces. Seems like the rolling forces you note are almost always shared, and almost never exclusively on one axle.