Re: XF500 6-speed vs STi
Any time you have issues with traction in a front engine rear drive vehicle it is a matter of weight transfer.
Also if you had to rely on friction, no 1/4 mile could be faster than 9 seconds. What you need are wide sticky (why there are burnouts) tires to use fractal surfaces to exceed 1 gee. Slicks used very low pressures (wrinkle wall) to maximize the contact patch on launch and were like an extra gear as they grew at speed. What I have found works best is a rim width just about the same as the tread width to keep the tread flat.
This is why back in the day before really good tires (and breakouts which destroys the whole purpose of a race) you jacked the rear end and used 90/10 shocks so that when accelerating the car would be level, and a slider clutch (before the Ramchargers made automagics popular) to transfer the weight back.
A characteristic of a really fast car was that it seemed to raise straight up when what was really going on was that the slappers were pushing up on the body and down on the axle.
However an IRS (like a Corvette or Jag) is different. The diff is bolted to the body so it does not wind. In this case you start with everything lifted so inertial forces are loading the rear end. Lowering is great unless you are in a drag race.
Just some thoughts but physics have not changed, just tires have gotten much better