I have an E92 335i and E70 4.8i.

Comparing both engines, I love the fact that 335i reaches its max torque in low rpm (less than 2000 rpm) and holds the torque for a long rpm band. You really don't need to down shift for fast acceleration, just step on the throttle. With 4.8i though, you need to stay in high rpm to get the instantaneous kick. Weight difference of the two vehicles of course have an affect on this but the main difference is the engine characteristics in my opinion.

With the low max torque and high torque bandwidth, I believe inline 6 twin turbo would be a good fit for X5. MPG savings is a plus (I get around 9 mpg with X5 and 16 mpg with 335i)

Not gonna happen. Not any time soon, anyway.

BMW wants to clearly differentiate the X6 from the X5. That is why BMW has no plans to put that bi-turbo 3.0 liter on the X5... it would cannibalize X6 sales, and minimize their strategy to make the X6 sportier and more performance-focused.

Juan

I read that the next-generation X3 will get a twin-turbo I6 variant when it comes out for model year 2011.