Just thought I'd throw out a few comments, as there seems to be some confusion on two issues:
Polar Moment of Inertia, and
Fore/Aft weight distribution
I think it's a bit confusing to label front engined cars as intrinsically understeering and rear engined cars the opposite. Rather, nose heavy cars tend to understeer, rear heavy the opposite. Naturally, the engine placement tends to determine the relative balance of the car. However, as in the case of the E36 M3, the car is essentially 50/50 weight balanced, and is not an understeering car with equal sized tires front and rear, although it is front engined.
On this issue of weight distribution, the heaviest axle tends to operate at the highest slip angles when cornering. This difference in slip angles front to rear determines understeering/oversteering attitude. In rear heavy cars like a Pcar, this is compensated for by increase rear tire/wheel width and decreasing rear roll stiffness through tuning suspension tuning. In this regard, notice that the Elise, which is rear heavy like a rear engined Pcar, runs no rear sway and has a 50 to 30mm delta in front/rear tire widths.
As to PMI, two cars can have the same exact weight, and same exact weight distribution at the wheels shown by corner scales, yet have vastly different PMIs. The closer the distribution of mass to the CG, the lower the PMI. Low PMI cars, like my last car (Honda S2000) as well as the Elise are quicker to get away from the driver, but also quicker to get back in line. In short, as stated above, low PMI cars have quicker reflexes, and drivers need better/quicker skills to deal with them at the limit.
Dunno if that helps or not, but thought I'd jump in and play.
