Here is a good post from rennlist regarding what happens when changing offset. Also note that lower offsets can change the spring rate and thus inducing a softer ride or a bit more roll. Leverage being the culprit.
From rennlist:
http://m.rennlist.com/rennforums/showthread.php?t=626833&styleid=21
"5mm I wouldn't worry about, but trying to increase the track width by 20+mm will significantly change your scrub radius and make the car a real handful on turn-in at the track.
For street driving not a major problem, but I wouldn't drive the car approaching the limit of adhesion at the track. Having said that, it depends on your skill level, risk, braking point/style, and corner entry speed.
IMHO, do the job right and move the front suspension to the outboard mounting points
To understand scrub radius you irst have to understand SAI -steering axis inclination. The steering axis(SA) is the line between the top pivot point of your hub and the lower ball joint of your hub. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. The angle between the SA and vertical is SAI, add static camber to that and you have the included angle.
The scrub radius(SR) is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. If these two lines intersect at ground level, then you are said to have zero scrub. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively.
older 911s have a small (+) SR, from 964 up they have a small(-) SR
the point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability.
The bigger the scrub radius the more kickback there is at the steering wheel. If you change to wheels w/ larger o/s you will feel the increased kickback at the steering wheel
If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.
Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly(as used on 911 front) typically performs well with a lot of SAI and caster(sound familiar?), a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.
As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.
abs equipped cars want a little (-) SR to allow for uneven braking forces between the left and right front wheels
