9000 Brake "Upgrade"

Some facts:

• 9000 calipers are 3.6 pounds heavier per corner -- that is, caliper and rotor. (This can vary a bit depending on which rotors you have.) This is all unsprung weight.
  
• 9000 calipers and Aero/Super Aero rims add 43 pounds to your car, all of it unsprung and a lot of it rotating mass.
• The heaviest 9000 rotors, by Balo and Brembo, weigh 14.95 and 14.88 pounds respectively; meanwhile, the lightest rotors, by Zimmerman, weigh 14.5 (their kick-ass cross-drilled rotors weigh 14.0 pounds). The heaviest 900 rotors, by Brembo (which is OEM, I believe), weigh 14.2 pounds, whereas the lightest, by ATE, weigh 13.44 pounds.
• The stock donut spare tire does not fit over 9000 brakes: if you get a flat up front, you'll have to swap a good rim from the rear to the front, then put the donut on the rear.
• Installing 9000 calipers requires an adjustment to the master cylinder to compensate for the longer pedal travel. This travel is due to the larger piston size in the 9000.
• Installing 9000 calipers and rotors requires the use of washers on the caliper bolts; without them, the bolts will rub against the rotors. (900 rotors are 278mm x 23.5mm thick, while 9000 rotors are 280 x 25mm thick. If you purchased "9000" rotors that are 23.5mm thick, then you were sold 900 rotors being sold as 9000 rotors.)

Read more on brake "upgrades."