I was curious so i looked this up. I copied these the from the tirerack and prodrive websites. You should call prodrive and luke at tire rack. They may be able to answer your questions.
"When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly."
"All Prodrive wheels are specifically designed for high strength and OEM fitment, for maximum performance and wheel-bearing life. Correct offset wheels ensure no increase in steering kickback, torque steer or tramlining."