An engine is a big air pump. More air in, more air out equals improved performance. It is as simple and basic as that.

More air in as long as more air gets out combined with enough fuel to create a proper mixture is first and foremost the basis for increasing torque and horsepower. This is true of any engine, naturally aspirated or forced induction. You can improve the performance of a lawnmower engine by reducing the back pressure. The stock exhausts of manufacturers don't pump up torque or horsepower, they reduce it.

Good headers are designed to have the proper tube size, length and collector to achieve better performance where there is the most benefit. If the design is good both TQ and HP will improve where and when it is best for that particular application.

Manufactures limit the tunes so they are in a safe zone for a reliable engine that stays together, generates acceptable mileage and meets emission requirements. Aftermarket chips, even used on a stock engine are more radical to achieve better engine performance and should be one of the first consideration to improve timed from A-B. The results are increased TQ and HP throughout the RPM range. Further, in the case of some aftermarket providers, such as Dinan, the tunes are specifically designed to work with their other other mods, such as low restriction exhaust, airbox etc. Increases in torque get you going faster so the goal is have the curve start higher, peak higher and fall off slower than stock curves. HP curve goal is to start higher, generally ramp up faster and higher to the optimum crossover of the curves very near where toque starts to decline. After TQ gets you going and then maxes out, the horsepower needs to keep you going.

If the only change is a more air in and out the TQ and HP curves will be similar to stock curve but be higher at all RPMs. They should peak at a higher RPM because the engine has the air flow needed to continue to generate more HP at higher RPMs when it didn't when stock but they you should never accept less at lower RPMs. In the case of an X5, I would never recommend a mod that reduces low end TQ or HP. never settle for that, don't have to to do so. Because of the weight of an X6, got to improve HP and TQ across the RPM range and increase the redline to use all the benefit of the added airflow. Because of that weight and all wheel drive improving the torque curve should take priority.

As you said, street 'races' are short. Loss of torque in those circumstances is not the way go. Until a good launch is not possible due to excessive tire spin or some other component can't take the pain, more TQ to get you going, especially with a heavy vehicle like an X5 is the best route. Except in a few circumstances such as high speed runs or something like oval racing where RPMs are always close to redline most of the time, losing HP or TQ from a standing start, light to light, a quarter mile, likely a mile will result in a higher ET. Lots of horsepower sounds good, lots of torque is what matters most.

The best mods to get the most out of a particular vehicle are exclusive to that vehicle or model if they are both the same specs. What is successful on one vehicle can be a disastrous or ineffective on another. It's interesting to hear the results of mods to other models like an E39, but we must be very careful drawing a conclusion it is the best route with any X5.

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