Agree that it's not big in terms of road tax but currently none of the X5's pass the emissions test and as the 3.0d is so close it would be relatively easy to achieve. Whilst it's not such a big deal for the road tax it is potentially a huge deal in London with the Congestion Charge. If it is being used as the threshold in London there is every chance that it will be used in road pricing schemes across the country will will no doubt be introdcuced during the life of the E70. If BMW do "tune" the E70 to beat the 225g emissions test it is potentially a big competitive edge they have as many of the competition don't have engines anywhere near passing the emissions test.

Here's a comparison
ML 320 CDI 249 g (ML280 is identical)
VW Tourag 2.5 TDI 267g (3.0 TDI 278g)
X5 3.0d 231g

MY BMW dealer thinks that BMW GB are looking quite seriously at this.