They why would you comment on what it's like to go downhill on a tandem?

I've got a Koga TwintySix, a trekking tandem, fully loaded it gets up to 180-200 Kg (two adults+camping gear, the 200 Kg is the maximum allowed load), the brakes are super good and yet, by the time you reach 70 kph downhill they are no longer capable of stopping the bike, safely or even at all.

A drag brake is a must in a situation like that or your brakes will start fading within minutes during speed reduction (short bursts of braking to reduce your speed to something safe and allow for ample cooling), and long before you really need them.

My comment was around experience of a reasonably heavy bike and braking down steep hills.

Like I said, its about technique, if you are getting up to 70kmh with bad brakes then you have bad technique.

The braking energy dissipated goes up square vs speed, so in that case you brake earlier and keep the speed down.

With that said, I have no doubt a large rotor with out heat soak can easily stop a 200kg bike from 70kmh to zero.