I twould over read by about 9 percent on the std wheel and tyre combo, but as the rolling circumference grows, the different starts to diminsh... As I said, the speedo can be be recalibrated cheaply, so shouldn't put you off.
It really livens the car up, especially off boost. Best thing I have done to mine!
I found a Brand new CWP in Norway, for 100 quid, couldn't say no!
A used diff will be sweet, as the 2.3 only knocks out 140 odd horses...