Here's an idea that might get you a relatively cheap data point before going through the full swap to see what you get ...

Put on a set of wheels/tires that have a 5% (or whatever exactly you're aiming for) smaller diameter, and take some test runs on those. I can guarantee swapping tires is easier than swapping differentials. And the results will probably be more relevant and accurate that anything you could get using a formula.

0-60 change based on differential ratio change
 

That is a perfect test case as long as can find a set that will fit over the bigger rotors. 5% smaller radius will get 5% higher torque-arm force to the road.

Re: "getting winded": the engine of course will get winded 5% sooner. The car will go over the power hill and hit redline 5% sooner in every gear. Part of the mystery of will you actually achieve faster 0-60.

When the torque band is wide enough the engine may do better with taller rear end. The smart money is on trying a tire with 5% smaller rolling radius. Get some take-off end of life tires or such.

My OBD app does automatic 0-60 and quarter mile timing you just have to stomp on the gas or does all the timing using the accelerometer to start and OBD to measure the speed. ( Of course you would have to account for the 5% wheel error).

That reminds me: both of our X5s have significant Speedo error they read 3-4 mph high at 70mph. Eg at indicated 73 I'm actually going 69-70mph.

The OBD knows the correct speed it's not from changed wheel size etc so I recommend learning on your car what Speedo error you have and read the obd data and confirm it is correct. (I usually clock a measured mile on the freeway with cruise to get my measured mile) GPS will give you a very good estimate of actual speed as well.

I'm going to make the change. I am positive it will improve 0-60 time, it's just a question of how much. If I have problems and the change is worthwhile I'll sort them out. I have done several mods on my 4.6 where I have had no benchmark for results or nothing to base it off that someone else has done. If it is a mistake it certainly won't be the first time I was wrong about something intended to improve performance.

This has been very worthwhile to help me sort out why I have been procrastinating. A major part of it was the beyond terrible experience I had installing the Seicane audio system. It still won't play DVDs. Going to audio shop this week to see if they can sort it out and get all behind me. Thanks for all the input.

My ¢¢: very much agree with your decision I would definitely do the same. Even if I was forced to drive 72 vs 75 to have the same RPM as I used to have (I have misophonia and can not tolerate certain sounds) it would totally be worth it to have the jump on somebody in a Corvette from the late 90s. (or really 99% of any car on the road: hell even sub 1L motorcycles).

Are you talking about your M sport or the other (you have two X5 now right?)

5% works out to 25' in a block. If you lose half of the torque gain to drive train variables you'd still be nearly an additional car length farther in a block vs your start point. Think of recent red light showdowns where it was neck n neck or the other guy was just barely sneaking away this upgrade will net you something like a car length per block. I didn't look at time to distance chart but I suspect you'll have already passed 60 mph by that point.

"Pics or it didn't happen". We would love to see a dash cam video

Diffs are for 2002 4.6.

All I have done on the X5M is buff and wax it, add low restriction air filters and made it so exhaust flaps are open all the time. We are taking it on vacation soon so once we get back I will add a tune. After that I will decide if I want to go further with maybe meth injection and downpipes. Not going to do anything radical as it is my wife's daily driver. But, I can't do stock.