When one makes the decision to do performance mods they should also expect there will be some problems, less reliability, more maintenance and at some point of improvement they will find the weakest link. That doesn't mean that any of those things will happen but go in accepting that could be the case.

In your case, start with a compression and leakdown test. If it doesn't pass don't do anything until it does. When it does, do some research on what other owners have done with the same engine. It is wise to never break new ground. Do the same mods others have done, same brands, no differences. Safest route is to research only the same model and year X5 as you have. I don't think you will find much. It is fine to research E46s and other models with the same engine but that info has to be applied to the drivetrain limits of your vehicle.

Rule of thumb is that .1 increase from 0-60 will cost $1000 or more. I always start with low restriction exhaust and cold air box. That will improve the performance of even a stock engine some. Butt dyno feels like it is more than it actually is and it improves the sound of the engine and exhaust, more fun driving it. And, other mods will want more air in and more air out anyway. Next I would research to see if someone like Turner motorsports has a plug and play tune. I would be surprised if you find one for an X5 but I expect you can for an E46. Another option is going to a local custom tuner. Stock tunes are always conservative and HP/TQ curves are aimed at efficiency more than performance so it pretty easy to bump it up. My guess is that adding 50HP with whatever TQ increase comes with it will not exceed the limits of the drivetrain even if it exceeds the BMW max spec but that's something to spend some time researching. 50HP at the crank will only be about 35HP at the wheels. I recently added 150HP to our X5M with just a plug and play tune. If I break something, oh well, s$%t happens.

If you want more than 50HP, a turbo, nitrous or a supercharger, and be safe, you would need to address the internals--that's expensive, especially if you can't DIY. You could buy a newer X5 that has more than double the performance for less than that expense.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas