I recently went to Jason at BWChipTune to get my X5 E53 2005 model remapped on feedback from various BMW forums. Jason is a top guy who really seems to know his stuff. His rates are also very reasonable compared to some of the others quotes I received from competitors.
I was looking for a more fuel economic tune as opposed to much more power but as I left Jason's workshop, the power difference and smoothness was immediately felt. The car was still a little sluggish taking off with a one second lag between putting the foot down to taking off but once it kicked in the drive was more like a sports coupe than a 4x4. Being used to your traditional 4x4 off road vehicles, I remember thinking that a large car like this should not be able to go so fast.
Still, the test was on fuel efficiency where a week later I went to France and completed 2,300 miles in 9 days. I set my consumption monitors for the journey and was mainly on the motorways sitting just above 80mph (around 2,650 rpms) with some country road driving in between. I was driving fairly conservatively and not flooring it very much where overall I got 31.7mpg. Admittedly, this was not much better than I got on my last trip to France over Christmas but I have more power.
In summary, if you are looking for fuel economy only, the gains in conjunction with the fun of putting your foot down will probably not see any improvement as you might as well enjoy the additional acceleration if you have it!
I am joining the fans of BWChiptune and I would recommend going to Jason - you won't be disappointed.
Thanks Jason for pimping my ride :-)