You will probably get a variety of people telling you to use the search function. To be honest when I did I got a variety of different assumptions on the 8mm mod. So I went and tried it. Basically you are adding a riser to the rear hub that raises the position of the ball stud height. Using the B4 carbon risers, the "stock" positions available if you look at pics become the inner ball stud position on the hub (as far as position goes) and even 1 spot inside more.

From what I saw by trying it, it appears to generate more rear traction, this was with an inner 5mm to 7mm ball stud height position and the outer location on the hub riser.

From what I have heard, people who use it on higher traction conditions had traction roll issues.

what I didn't like about the way I did it was that the position of the ball stud on the hub is more inside that I wanted, shortening the length of rear link. This seemed to make the vehicle square up a bit more than I wanted.

i think the exotek setup may be a better way to go if you really want to use that kind of mod.

Personally, since I would have gone to this mod for more grip in the rear, the M2C toe in blocks in the 3.5 degree setting gave me exactly what I wanted for grip.