i have the 3.0 2003 X5 and i also had a Porsche CaymanS.
0-60 times for are 8.3 and 5.5 respectively. Porsche obviously feels quicker off the line.
Top speed is 125mph and 177mph. Never broke 120mph in either.

X5 is not a sports car. The simple fact that you ride so much higher than a Porsche gives you the feeling that you are moving slower. The lower down to the ground you are, the faster you feel.

I would have to check my speedo on the exact same highways to make sure that I was going 80mph in the X5 (which I was but felt way slower) while in my Porsche I felt dangerously fast at the same 80mph.

3.0 is enough for me, but I guess if I wanted more performance/less mpg then the 4.8 would be a good single car that encompasses speed and comfort and room and safety for roughly $7000 more