I have a set of wheel spacers on my rear wheels of my 2.8. So far I like them very much. They help to push the rear wheels out to align with the rear fenders to get a more aggressive look. They also, in my "butt feeling", improve stability and handling of my Z as a result of wider rear track. It is one of the best upgrade I've ever had and I highly recommend to others. Wheel spacers should be one of the 'must have' upgrade along with windscreen, IMHO. Here are some pics:
Again, all improvements are based on my "butt feeling" with no scientific or track performance proof. All comments and experiences are welcome.