However, your speedo will still say you are doing the higher speed, as it takes the pick-up from the gearbox, so if you changer the final drive, it will not know this. So be aware that your speedo will also be over-reading by probably 12-20mph (depending on how far it over-read in the first place before this modification).
interesting, my mate has a 2wd saff and the speedo is reading similar but low what diff is that likely to have???? as its had a recon or 2nd hand one not long ago cant tell from the receipt.