Thanks for those trying to be helpful. I do appreciate that there are a lot of genuine people here.

Weasel raises a great point about warranty, as I still have warranty would need to think through. For those that took the time to look at feedback from the seller (rather than just making a joke out of my original question), there are a number saying that it made a great difference on other vehicles - as well as some who said it didn't, hence my reason for asking re the X5. Also thanks to mrbmwx5, there are certainly some strong supporters for the sprint booster. So even though it is more expensive, sounds like a better piece of kit.

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2004 3.0i X5 E53