German Tuner IMSA upgrades the S63 AMG to whopping 720HP; they haven't fiddeled with the exteriors except but replaced the standard rims with their own 21-inch forged lightweight alloy wheels wrapped around in 255/35 R21 front & 295/30 R21 rear tires. The V8 5.5-liter twin-turbo engine has gone through some software tweaks and received new sports air filters, modified x-cross central muffler for the exhaust as well as optimized turbocharged outlet, intercooler radiator and an improved air flow rate.
All these changes have increased output of the engine from the series 585 HP (430 kW) and 900 Nm (664 lb-ft) to no less than 720 HP (529 kW) and a peak torque of 1,080 Nm (796 lb-ft). Performance details have not been disclosed but it should be significantly quicker than the stock car which needs 3.9 seconds until 62 mph (100 km/h) and is electronically-capped at 155 mph (250 km/h).
IMSA says in the future they will roll out more tuning packages for Mercedes-Benz models, especially for those wearing the AMG badge. The S-Class Coupe will soon receive some power upgrades along with a custom aerodynamic body kit.
Source: Official: 2014 Mercedes-Benz S63 AMG by IMSA