About rim width, it all depend what tire you are using.
The friend with the 500+ bhp S3 has 205mm wide rally slicks. So, for him, 8 inches rim width is worse than 7.5 inches (as he has on his OEM rims).
Also for 17 inches, 7.5 is ideal for tires like R888 and Kumho v70A (225mm wide), while BFGoodridge say 8 inches rim width is ideal for their 225mm wide tires.
For 18 inch rims most manufacturers say that 8 inches is the ideal width for 225mm wide tires. BUT there are slicks that are 205/215 mm wide which need some thinner rims
.
So it all depends on your tire choise...