While details are still nascent at this point we do have enough to build ourselves a semi useful comparison of the 12C and the 650S.
First things first, the motivation. 650S uses almost the exact same 3.8L Twin Turbo V8 as the 12C, except that in the 650S the engine cracks out an extra 25 horsepower. 650S is good for 616 hp and 500 ft-lbs. Gains come from alterations made to the cylinder heads, new pistons and improved cooling.
Aerodynamics are also virtually identical. McLaren quips that the drag coefficient of the 650S is identical to the 12C's except McLaren does say that the 650S makes better use of the airflow. What they mean by that is the new aero kit will privde 24% better downforce at 150 mph. Not insignificant but these vehicles will spend very little time North of 100 mph. Just saying.
Anyways back to the downforce, McLaren CEO Mike Flewitt had this to say:
"The newly designed front bumper improves downforce and steering turn-in. The high speed handling balance is now even better..."
The McLaren Airbrake (read spoiler) has been tweeked for the 650S vs the 12C to offer even greater levels of downforce. The improvements are simple really, when the 650S feels it needs extra downforce the spoiler will comply. Simple really.
Upgrades to the 12C's suspension we're made as well for the 650S. The 650S sports a 22 percent stiffer suspension, front and rear.
Gear boxes changes were simple as well, McLaren simply recallibrated the 7 speed DCT to shift even quicker.
The last of the significant changes comes from the stopping system. The 650S uses standard carbon ceramic brakes and new brake booster that promises to keep pedal power linear. Coupled with the new AirBrake and a recalibrated ABS system the 650S should be a touch more engaging than the 12C.
Add more as you find it