We used the 6-bolt XT RT86 ICE rotor last year with great success.

Here is a performance comparison article covering the ICE rotors last year. The writeup in that link shows just how effective the rotor can be under what they call "normal usage". The numbers may not cover heavy tandem usage, but it is a quantitative comparison nonetheless.

The latest and greatest ICE rotor out on the market is the Saint RT99 which incorporates heat sink/vents. Unfortunately it is available only with centerlock (not 6-bolt) and there is no way (AFAIK) to mount a centerlock rotor to a 6-bolt hub.

For this new version, Shimano claims:

• Additional 50°C decrease in operating temperatures over previous Ice Technology rotor
• Braking and performance benefits include 20% less brake fade, 20% increase in stopping power and 20% longer pad life
• Available for center lock hubs
• 3-layer clad rotor – steel / aluminum / steel
• Weight is 173g, which is 21g lighter than the last 203mm rotor.

With a quick search on pricing, I found ranges from $129 for available now (Jenson), to $81 in May (Chainreaction).

Note again, there is no way (AFAIK) to mount a centerlock rotor to a 6-bolt hub.