I noticed the same thing too on my 2005 X5. I recall the dealer telling me when I bought the X5 that the engine would feel peppier at around 60k miles. I dismissed that at the time as an odd statement. He turned out to be right.
2007 3.0si w/Sport Pkg, Navi, Leather, Strut brace, garage door opener and delete airbag warnings on sun visors