It is definitely not slower, though valvetronic may make it feel that way.
315hp/324tq, the rpms are nearly the same for both motors.
The older 4.4 utilized a 5 speed automatic compared to the new 4.4's 6 speed, which may loose some instant pull feel off the line do to gearing.
The new 4.4 is only 3 ticks shy to 60 from the beast 4.6, which isn't bad of course it isn't any 4.8.
Automobile magazine had the following:
2001 4.4 at 0-60 in 7.1 and 1/4 mile in 15.6@89mph
Motortrend had this:
2004 4.4 at 0-60 in 6.9 and 1/4 mile in
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Seems to me the latest rendition of the X5 4.4 is guided more for fuel economy then "power feel", even though numbers say that the new 4.4 is faster and 315hp as opposed to 285hp...probably is.