If no turbo for X5 for ever, for years, we'll see a 3400lb 335 having 300lb-ft torque, while a 4800lb X5 3.0 having only 220 lb-ft. It doesn't make any sense to me. It is X5, not 3, who needs more low-rpm torque badly.
If the 255hp N/A is only for MY2007 (i.e., turbo in MY2008), it doesn't make sense either. In MY2006, when everyone (3, 5, Z) got the new 255hp 3L, X5 stayed with the old 225hp version. Why? because MY2006 is the last model year for X5, it's not worthy putting a new engine in just for 1 year of production. Samething, it's not worthy putting in the 255hp new engine just for 1 year.
The only reason for this 255hp new engine's 1-year tenture is, I think, BMW is not 100% confident with the new bi-turbo engine yet. They wants to try it first on 3, fix all the problems, then put it in 5, X3, X5, etc.