The real reason for all the overkill is mostly marketing. They need all the "street cred" they can muster to go up agains tthe AMG SL55, M5, M6, etc. etc. etc. 8 piston liquid-cooled brakes sound way cooler than standard 6 pot brakes.
But I agree; in that segment, big fat rotors with good strong 4 piston calipers engineered properly would be about perfect for 99% of buyers.