Raw torque numbers do not solely define how a vehicle will tow.
Now, I will introduce myself so you can know that I know a few things about towing, sometimes.
Me
I have a current CDL A license, as I am a heavy truck and equipment mechanic (currently running a shop so not really a mechanic anymore), I have driven a few class 8 (80,0000lbs capable) setups that have had trailers, I have personally spent many hours behind the wheel of HD pickups with cars in tow, and have a personal affliction where I love HD pickups (though this love has been strained in the past 5 years by the HUGE inflation of prices in the market, I liked heavy trucks before anyone cared about them)
My Vehicles
I currently own a 2011 BMW X5 xDrive50i, that features the 4.4L twin turbo BMW V8, it makes 400hp, and 450lbs ft of torque. My preferred vehicle to tow with is my 1984 Ford F250, it has a 6.9 liter diesel, I have installed a non inter-cooled non waste-gated (I have disconnected the OEM one) single turbo, that makes 10lbs of boost +-2, and when towing will occasionally flash to 12-13lbs. This is an extended cab long bed 2wd truck, it runs 16" wheels with E rated tires.
What I have towed with this particular truck and how far plus other tow stories
I have put my 7.5foot long toolbox in the back of the bed, and that weighs, if I had to guess, somewhere in the neighborhood of 4,000lbs
I have pulled many cars with this, most notably my 300zx to someone I had sold the car to in Tennessee.
The heaviest load I have ever pulled was and always will be my wife's 2005 Jeep Grand Cherokee limited with a hemi V8 that truck weighs 5,000lbs, I hauled it with a steel surge brake U-haul trailer, and they typically weigh around 2000lbs. It pulled it great, like everything else it has ever pulled, she is awesome
Compare and Contrast
A 2017 Cadillac Escalade has a 116" wheelbase and weighs 5780 lbs, with an overall length of 203.9"
A 2011 BMW X5 xDrive50i has wheel base of 115.5" and weighs 5379 lbs with an overall length of 191.9"
An Escalade is only rated to tow 8500lbs with 2wd, when you option the AWD/4WD, like 89% of the world who buys these trucks will, the tow ratting drops to 8100lbs. The reason why this is rated that way, is you have a body on frame truck, with big beefy springs, big beefy suspension. You don't noticed the big beefiesness because the Escalade sits higher, so those big beefy components have a longer travel and it does not adversely take away from the ride quality. the x5, though quite capable, is a unibody construction. These uni bodies can be strong, but in the case of BMW is based off of a car. This means they have relatively beefy components, but they are more towards car components then truck, so you lose a little weight carrying capacity, and gain a sports car like drive.
Also, I don't think you understand how much girth an 8,000lb load is, I would not, under any circumstance tow that with an x5. 8,000lbs is also near the limit of off the hitch loads. Typically once you jump into the 9k and above range, your trailer to tow said load turns into a gooseneck or 5th wheel.
So to summarize, no, I would not tow beyond 6,000lbs, yes your load of a 3 series will be fine for an X5, that car with a typical 2,000lb car trailer will put you right at 5k +- a few, and any x5, even the 6 cylinder, should be fine to tow that. Don't get snobby on the power, I once towed a Camaro with a early 80s Chevrolet Silverado with a v6, that probably made a worn out 100hp. People these days just want to haul everything at 80mph, but sometimes slow and steady gets the job done with no fuss, see my F250 above.
p.s. I think we spoke at the Aktiv DIY meet about this?