[27litres]

Interesting discussion about ground clearance.
The actual height of the towbar off the ground when fitted is only half the story. It's all very well that it's 2.5 spark plug boxes off the ground, but that isn't going to stop it striking the ground on occasion.
You need to look at what is called the departure angle (4x4 lovers will know about approach and departure angles).
Basically, you draw a line from the bottom of the rear wheel to the lowest part of the car to the rear. The larger the angle the more clearance the car has. The car design will put the angle somewhere in line with the lowest part of the rear bumper or rear most section of the exhaust depending how far that sticks out the rear of the car. That line will rarely strike the exhaust forward of the rear bumper as that should be designed out by the manufacturer.

Anything such as a towbar that sticks out below the bumper level or out to the rear significantly will lessen the departure angle and make ground strikes more likely.

I have an AutoHak detachable towbar on my D2. It sticks down below the bumper about 3" similar to Steamships (Bosal? PF Jones?). And with the swan neck attached it mounts very similar to Steamships as well (maybe a slightly higher angle), making the lowest point on the departure angle the bottom of the hitch mount.
I get occasional ground strikes (in a S8) over steep driveways and speed humps and the like. It doesn't happen very often.

I also have (but have not yet gone about fitting) a Westfalia vertical mount factory towbar. The ultimate in ground clearance with the swan neck removed! Obviously I bought this used (Westfalia haven't made this model in 10 years) and the swan neck shows evidence of ground strikes - even on the factory bar!
Before locating the Westfalia bar, I also approached AutoHak's Australian distributor and bought the parts required to convert the detachable bar to a vertical mount bar using parts from their D3 model.
So I'm now undecided which path to head down!