According to Car Magazine, the 4.4-liter twin-turbo V8 that debuted in the BMW X6 is coming to the 2010 BMW X5. That'll give the X5 408 HP, a 155 MPH top speed, and a 0-60 time of 5.5 seconds; just a hair slower than the X6.
Expect it to also gain an X6-style front end with big air intakes and a sculpted hood. There's no word on whether or not Dynamic Performance Control (the technology that really gives the X6 its magic) will trickle down to the X5, but we wouldn't rule it out. The new engine could also bring with it the "is" badge, like the previous generation X5's 4.8is range-topper.
We've driven an X6 equipped with the V8, and while its performance is indisputably impressive, we preferred the slightly slower twin-turbo inline-six. The V8 weighs the X6's nose down too much, while the awful gearbox and turbo lag make exploiting the engine's full potential frustrating. Based on the same chassis, we can see no reason why the same won't be true of the so-equipped X5.
[Source: Jalopnik] 2010 BMW X5: 2010 BMW X5 Spotted With 4.4-Liter 408 HP Twin-Turbo V8 From X6