[The HACK]

This is one area that I haven't really delve into, since 1) I don't run spacers for track and 2) The offsets I run is typically within a few mm of factory. There's a couple of points I'd like to contribute though.

First, the talk about scrub radius only affects the front end, but increasing/decreasing offset also affects the rear. One thing few takes into consideration, is that by decreasing the negative offset, you are in effect decreasing the wheel rate if you do not change the springs, since the "lever" effect is increased. Especially on the rear, where the springs are typically mounted further inboard than the front. This is one of the reasons why I never really was all that interested in messing with the wheel offset, and unless clearance is an issue I typically stay as close to OEM offset as possible, since the effects are somewhat unpredictable and non-linear...The same decrease in negative offset front AND rear may actually result in increased understeer.

On my E30 318is, I had to run a 5mm spacer up front to clear the brakes with my generic Civic rims (when the brake pads are below 5mm the wheel fits fine. Brand new pads would have the caliper catch the back side of the spokes). When I took it to Sears Point it actually behaved better than I expected, so even a small change in offset could have some pretty dramatic effect. But it is definitely one of those things where, to me, the goal is to fit the biggest and widest set of tires and rims possible, not use offset to fine-tune my suspension geometry.