Yeah...apparently some people have snapped the QR axle. I guess it makes sense because the 8in rotor does produce more torque since the caliper is located about an inch further away from the hub as opposed to a 6in rotor. I'm thinking of going with a 185mm rotor, which should be a good compromise. Does anyone know any manufactureres that use 185mm rotors...all I know of is Avid.
Hayes now makes 7" rotors and brackets, not sure of stock or availability yet anywhere. Both Avid and Hayes 8" rotors are 203mm, both Avid and Hayes 6" rotors are 160mm. Avid 7" rotors are 185mm, I don't know, but would expect the new Hayes 7" rotors to be the same 185mm. Hope 7" rotors are 180mm, not 185mm, but you can get Hope Gothic replacement rotors in 185mm. Other than what I don't yet know about the new Hayes 7" rotors (185mm or 180mm), Avid rotors are interchangable, and so are the caliper mounting brackets.
Fox says you can run a 7" rotor with a QR dropout fork, not an 8". The other fork manufacturers may say something similar, but I have noticed that this does not stop bike companies from frequently OEM specifying 8" rotors on QR forks on many new bikes. The concern is that the additional force from the different fulcrum of the caliper mount with a larger rotor could cause the front QR wheel to move, but it would not come out because all QR fork dropouts made today have "lawyer" tabs or a recess around the clamping surface to eliminate your wheel coming out, even if loose, which you would diffinately feel first.
I run 7" (Avid) rotors on the front of my Monocog ('06 TALAS 90-130), my XC hardtail ('05 Reba U-turn 85-115) and the front of my Cannondale cyclocross bike (rigid). All are QR, no problems. I bent the lowers on my '03 Fox Float RLC 100 that used to be on my XC HT with a 7" rotor, appears to be twisted in the exact direction excessive brake force would twist a lower. Had a 6" rotor on the front of that one for the first two years I rode it, so not sure. I run 8" rotors on the front of two other bikes with 20mm through-axles, and a 7" Avid rotor on the Fox 36RC2 on my BLT. I thought of swapping out the front 7" for an 8" rotor on the BLT because I can, but I don't need it and it would just be heavier. I do notice the difference with 8" rotors on my 43 lb. Bullit for what i ride down on it, but doubt I'd notice a justifyable difference from swapping the 7" for an 8" front on the 30 lb. Blur LT, I ride it as a FS trail bike, I don't bomb down a mountain at 40+mph on that bike. And while climbing techy stuff on that bike (where it doesn't matter what brakes you have), I'd be more likely to smack a bigger rotor on a rock than a smaller one.
My 2 cents.