I have fitted quite a number larger turbos to 3.0Ds ,but mainly to the pre-facelift ones which came with the smaller Gt2056 and GT2256,where we upgraded to the Gt2260 from the later models.Only technical change we made to the Gt2260 was to fit a 360º thrust bearing to replace the original 180º bearing,which along with oiol starvation from a blocked breather,is a main reason for turbo seizure!This is the main bearing in the shaft of the turbine and compressor wheels.
For those that dont know,GT2260 ,basically means it has 22 turbine wheel and a 60 inlet compressor,a GT2056 will have a 20 turbine and a 56 compressor..and so on.
On 90% of the turbo upgrades we do,we try to keep the same size turbine wheel ,exactly because of minimal lag.
Mine is a pre-facelift and on mine I have a GT2267 which is basically a GT2260 turbo but with a larger 67 compressor ,off GT3067 turbo.Its the compressor that increases the air volume ,meaning you then have to fuel it and therefore more power.
This would also be a good "soft" upgrade for your facelift 3,0D but you could even fit a complete Gt3067 if you wanted to go wild,but the problem with X5 is the autobox,which doesnt like excessive torque ,and even the 615 Nms ,you have now is way above the 500 Nms standard!No problem with a manual box ,as we could reinforce it.The other problem with the 3.0D is the plastic inlet manifold ,and I popped one open after running 2.5 bar peaks.Have to make an aluminium one
Rather than stay of-topic here,feel free to PM me