Reporting back for the benefit of anyone who may be following. Even the Mercedes dealership had never seen one towing a horse trailer before (more below on why I wound up at there).
I spent the last two days roaming about on hills, curves, freeways, and city streets with the G63 and my empty Brenderup Baron L. I floored it uphill and braked hard downhill a few times. I drove across a bridge with strong crosswinds, and was passed a few times by loaded semis with my foot off the accelerator to check for sway* (none). Frankly I couldn’t tell the trailer was there. Of course, a loaded trailer will be different, but I have no reason to think the G63 won’t perform at least as well as my previous 4Runner V8.
That said: I firmly agree with everyone who has pointed out that you always want as much truck as you can get and more truck than you need, because what matters isn’t whether you can move, steer, and stop the trailer in good conditions, but whether you can move it quickly, stop it quickly, and keep it stable in emergency situations. If I had my druthers, I’d have a horsebox. Failing that, a semi or at least a 3/4 ton pickup. But if one is limited to an SUV and has a G63 in the garage, it will serve, with a trailer designed for dinky tow vehicles (Brenderup, Boeckmann, etc.). I wouldn’t use it, or any SUV, to tow horses in any other trailer. And I would never feel comfortable with less than a V8 or with an aftermarket hitch (you need the transmission and cooling features of the OEM tow packages).
As for why I wound up at the dealership: the Brenderup’s electrical plug is a 6-round, and the G63’s plug is a 7-blade. It turns out that not every adapter labeled “6-round to 7-blade” will do; you need to find one like the Hopkins multi-tow that includes the center pin on the vehicle side of the adapter or else “Nein nein nein das geht nicht!” and the trailer lights won’t work.
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*A note on sway: Brenderups supposedly don’t have issues with sway, but I had sway problems with my 4Runner V8 and my Baron L. I’m not the only one, either. After discussions with both Mr. Truck and Mr. Trailer, my working hypothesis was that the angled back of the non-L Brenderups contributes to the anti-sway design, and when you take that away for the extra length of the L version, you may lose some stability. So, I upgraded my tires from passenger tires to Sprinter tires (Continental Vancos), which have a “C” (European commercial) load rating and run at 55 PSI, but come in the correct size (205/65R15). It will take a few more trips to be sure, but I think that may be the answer.

