The E63 AMG in addition to the AMS ECU software also has prototype AMS Alpha downpipes and a prototype AMS Alpha catback exhaust. The McLaren of course is a Spider which means some added weight although the difference between it and the coupe is roughly 80 pounds. The difference between the MP4-12C Spider and the E63 AMG 4Matic however is roughly 1100 pounds.
A stock MP4-12C is pretty damn fast as it sits. A McLarenBoost member took their stock MP4-12C fitted with the 616 horsepower file to the drag strip and ran a 10.6 @ 135.
That should put the AMS Alpha 7 E63 AMG 4Matic performance into perspective considering the E63 edges it out with a 156.90 trap speed to the McLaren running a 153.89.
These are two very fast cars.