Overview
A caveat. I don't know a lot a about the inner workings of motor vehicles... 
I did a lot of research on any performance chips that might be available for the E70 4.8i (as I'd had performance chips in my previous 325is and 328i). There are a couple available in Europe for the E70 3.0si and there is one for E70 4.8i from Hamman (which I would classify as a reputable company). That requires 98 octane fuel so I had to forget that option (in the USA). So there is no USA option for boosting HP or torque (from reputable manufacturer).
I really wanted to see if I could improve throttle response though (as the HP and torque in the 4.8i is actually fine).
Sprint Booster's Throttle Enhancer was the obvious one as it's been discussed on many different BMW & MB forums. There are people that are very happy with it and there are others that have problems with it or claim that it falls into the same (useless) category as the "Tornado Air" (which supposedly increases MPG). The Sprint Booster price has come down to as low as $300 and is carried by reputable companies such as ECS Tuning, Turner Motorsport, Bimmerworld, Bavarian Autosport, etc. Some are offering a 30-day money back guarantee. Another product, that appears to be a similar to the SB, is the
G-Power BMW E-Response Module. I asked Jace Tuning why the product was $1,003 compared to the Sprint Booster but never got a response from them.
I decided to try the Sprint Booster and purchased that.
Installation
Unfortunately, installation was not as easy as the single instruction sheet showed. The instructions for removing the throttle pedal assembly were for a totally different model of car and did not work for the E70. After some searching, I found some instructions which helped. This is my version:
1. Remove floor mat
2. Flip up black plastic button at bottom of pedal with small screw-driver (see pic 1)
3. Remove allen bolt holding bottom of pedal
4. Lift throttle pedal up (from bottom) and wriggle it a bit while doing that
5. In pic 3 you can see the clip that was holding the throttle pedal in place (along with the allen bolt at the bottom)
6. I haven't attached any pictures of attaching the Sprint Booster install as those are straight forward and covered in the instructions
7. In pic 4, you can see what it looks like after the Spring Booster is installed. It looks ugly and there is no excess cable on the SB to tuck that up under the dash (as I had hoped to do and that others with other BMWs had been able to do). Maybe there's different model that has longer cabling -- I had supposedly received the correct SB for the automatic X5 though
8. In pic 5, I had temporarily put the little switch where you see it. There are three settings (light off = Stock, green = Sport, red = Race)
Testing
I put it into Sport mode. Pressed the gas pedal. The X5 jumped and lurched forward! I had to learn to drive differently. The throttle response improvement was FANTASTIC! I had to
gently touch the gas pedal and I got IMMEDIATE throttle response and only had gently depress it further to accelerate fast.
So this was a good thing for normal driving around town but that's not where I found the real benefit. It was on the freeway driving at 70 - 80 MPH and above. When driving at those speeds, overtaking was RIDICULOUSLY SMOOTH and RESPONSIVE. If I was in the #2 lane and wanted to move over quickly, between two cars, into the #1 lane, I just slightly depressed the throttle and got immediate and smooth response. None of the "mashing the pedal to the floor" and the resulting lurching.
So for freeway driving especially, I felt like I was driving a different vehicle
I hardly pressed down on the throttle at all and it was very comfortable to drive. There was no negative effect on my MPG in the time that I tested it.
Negatives
The negatives are
1. The Sprint Booster switch being right above the throttle pedal assembly
2. The red "Race" mode is unusable as the car jumps forward at the slightest throttle touch
3. The "Sport" mode or the green light does not switch off when the car is switched off
4. If another driver, who doesn't know about the Sprint Booster, drives your car with it in Sport or Race mode, they could easily be overly surprised with the throttle response and cause something bad to happen to the car's front end
5. It will most likely affect the warranty. But is quick and easy to remove -- only 5 minutes work.
Why I've removed it
My X5 has the
Active Steering w/Servotronic Power Assist option. This adjusts the front wheels' steering angle in proportion to the car's speed. In low speed, the front wheels respond immediately to small movements of the steering wheel (parking and driving in tight spaces is enhanced) while at high speeds, it gives more precise steering, great stability and more comfort (which results in it being very easy keep the car exactly in the lanes and less tiredness when driving long distances).
I found that the Sprint Booster
made my Active Steering no longer work 
I first noticed something different when in town; I could feel the steering (unpleasantly) different. On the open road, this difference became a lot more apparent. It took more effort from me to stay in the lanes (as I'd lost the stability enhancement) -- it was obvious that Active Steering was no longer working. Everything else seemed to be working just fine.
So I've uninstalled in and hopefully the supplier will stand by his 30-day money guarantee...