15" rims will limit you to 11.75 rotors if using stock parts. To lighten this up you can swap to FJM spindles to loose a few pounds and swap the stock calipers for Wilwoods using ARE adapters. This is about as light as you can get using stock rotors and you will loose around 7-10 pounds per corner and cost maybe $600.

If cost is no object, then a custom Wilwood 12.12 carbon rotor on a billet hub designed for a drum spindle and 4 piston racing calipers and adapters can drop 15-18 pounds per corner and cost you around $2500-3000.