There are formulas for estimating top speed. One of the factors that is a significant is the frontal area of the vehicle. The X5 has a high frontal area so wind resistance builds up quickly and to go just a few mph faster it takes huge increases in HP/TQ and a great distance to realize the gain.
A top speed discussion should be at least over a measured distance, better yet at the same place on the same day. Just to get a sense of how much horsepower it takes to go fast I am attaching a video of Corvettes at the Texas mile. Since a majority of us know the capabilities of a Corvette and it has such low front area it seems like a good example.
https://www.youtube.com/watch?v=o329ZFqnsEM