You might want to take Shimano's guidance seriously. The first set of brakes I put on our Ritchey Double Switchback were Hope's E4 "Enduro" mountain bike brakes. I suspect they are more powerful than any road bike brake. They are 4 piston brakes and they have a fairly large swept area. And I had 203 mm rotors. When I needed to make a panic stop on a 10% hill at a speed of 40+ MPH, there was smoke pouring off both rotors and the rear was very, very hot. To say that we had a brake fade issue is an understatement.
OK, blame me. What was I thinking? We were beat, the Scalloway Hotel was waiting with food and beer, the wind was blowing 50 MPH and it was in our favor, and the blind corner on the hill was a wide sweeper. Who would think there would be road construction around a blind corner and the flagger would not be visible on our side of the corner? So, 500 lbs of riders, tandem, and luggage overwhelmed the E4 brakes.
I contacted Hope and was told in no uncertain terms to put the V4 downhill brakes on the tandem. By the way, Thorn has a write-up about their brake setup and why they will only supply a tandem with the Hope V4 brakes. You might want to read it before fitting disc brakes to a tandem. I wish I had.
I now have V4 brakes and the vented rotor in the back with an IceTech in front. Trouble solved. If I ever have a hint of trouble in the future there will be a Wilwood Kart caliper in the back.