I have Shimano M760 6" disc brakes and am considering a DH trip to Costa Rica. The tour would last for 2 weeks and we'd be riding for 4 - 6 hours at a time on technical DH. Apparently, one of the peaks on the trip is 3000 m tall. I'm worried about hand fatigue and brake fade with my 160 mm (6") rotors. I have three questions:

1) is there a noticeable improvement in brake action when comparing 180 mm (7") rotors against 200 mm (8") rotors? Torque = Force x Radius so I expect to see a 12.5% increase in stopping power for 180 mm compared to 160 mm. Likewise, I'd expect a 25% increase in stopping force compared to 160 mm rotors. Is this it or are there other "real-world" considerations the experienced riders can enlighten me with?

2) Shimano XT adapters - where can I find some caliper stand-off adapters to accomodate the larger 180 or 200 mm rotors? I've been hunting high and low and haven't found anything for 180 mm. It would appear Shimano doesn't sell 8" XT rotors anymore.

3) I have an '03 Fox Vanilla 125R. Can I use the 200 mm (8") rotor safely on my fork? I only weigh around 150 lbs. I've read about the possibility of drop-out failures but I imagine there's quite a bit of safety margin to encompass the broad range of weights and conditions. Is this a marginal concern or is it beyond reason?