I am looking into the rs4 replica wheels and am wondering what the main differences will be (fitment and handling characteristics) of having an 8" vs an 8.5" wheel.

 

umm you're in the wrong section buddy ^_^

 

The new 07 RS4 wheels?

Well either way.... 8.5 if you get a 255 series tire you might rub but you could go 235 or 245 and be fine.
Wide will allow more power to ogto the ground with out loosing traction. Which shouldnt be an issue unless your stage 3.
Really its not gonna be the end of the world but i think you should put the extra meat under there just to have the security and extra handling.

 

thanks but what width would promote oversteer?

 

Um...aren't you planning to put them at all four corners? If you increase the size of the contact patch equally at all four corners, you will increase the grip equally at both ends of the car. If you are serious about wanting to "promote oversteer" by changing wheel size, you would want to put the wider wheel (with the wider tire) on the front and the narrower wheel (with a narrower tire) on the rear.

Here is a shot of Mark Daddio working that setup at the 2004 Solo Nationals in a Neon.

 

TRUE...if the center of gravity was exactly in the center of the car with power distributed equally at each wheel. I could be full of BS but it would seem that the car would have a lot more oversteer if you put more narrow tires on it. If you put bicycle tires on a car and then switched over to 12" (exaggeration), there has to be some diference as far as understeer and oversteer (thinking in terms of vectors).

 

Really if your that worried about .5 of an inch i dunno what goig on haha, yea true with the AWD if you toss more meat underthere, (in theory) you should gain more over steer just simply cause there is more traction to put to the ground. However other factors come into play..... Hard cornering mixed with acceleration will cause more stress on parts like Controll Arms, Axels, Tie Rod Ends, and the wider of a tire the more resistance youll get therefaore affecting those parts.

And on the side note the thing about different sized tires on front and back....
Not such a good idea... people do it. but idk quattro is quattro for a reason.