Its do differentiate between the low output N52's and the high output N52's...

See the 325i & 330i's all have the same 3.0 engine, its just that the 325 version is detuned and has a few exhaust modifications. While this didn't make much sense for the 3 lineup, it makes sense in the Z4 lineup (3.0i & 3.0si - which correlates with detuned and full power).

Although the X5 will probably never get the N52 3.0i, it's 3 liter engine will still be called 3.0si as to not create confusion across the line-up.

Why a detuned engine you ask? Apparently its was easier to meet U.S. emissions that way... (Europe has a true 2.5 liter engine).