Under normal driving conditions, the 4-Matic on sedans is slightly biased toward the rear wheels, which helps performance driving.
Mercedes provides the following in a late 2006 article on MBUSA:
"Under normal conditions, Mercedes sport utility vehicles distribute torque equally between the front and rear wheels. However, 4MATIC-equipped passenger cars come with a planetary gear set within the center differential that begins with 40:60 percent front:rear torque distribution (45:55 on the new S550 4MATIC). Mercedes-Benz 4MATIC SUVs (M-, R-, GL- and G-Class) employ a 50:50 torque split front:rear. To skilled drivers, this rear-bias all-wheel-drive system feels much like rear-wheel-drive Mercedes-Benz cars, in which subtle throttle-steering effects at the rear wheels can enhance handling, especially in spirited driving."
In another article dated 2005 (the 2007 S-Class 4-Matic system has been redesigned), Mercedes says "4ETS applies controlled amounts of braking pressure to spinning wheels and simultaneously increases the amount of power sent to the wheels with good traction. In this way the electronic traction control system achieves the same effect as three differential locks."
A Mercedes press release from 2000 says:
"Whenever one wheel begins to rotate faster than the other three, the traction control system momentarily applies the brake on that one wheel. Since applying the brake increases resistance on that wheel, the differentials naturally shift more power to another wheel. The Mercedes-Benz four-wheel drive system is one of the very few which will keep the vehicle mobile even when three wheels have lost traction. The 4MATIC system coupled with a standard driver-sensing Electronic Stability Program (ESP) enhances handling control in turns. If the car fishtails, ESP brakes the outside front wheel to help restore order. If the car plows, ESP brakes the inside rear wheel to help keep you on course."
I can't answer the question directly as to the amount of power shifted in each circumstance - what I can say is that after driving three models of Audi with its Quattro system (5000S Quattro, A4 1.8t Quattro, and A6 2.7t), in all kinds of weather, I don't think the Mercedes 4-Matic in my '05 gives anything away to them. I owned the Audis, driving the Quattros for about 3 years each, in Colorado, Baltimore, and Washington (DC), so had a lot of experience with them.
Mercedes has continued to refine the 4-Matic system; the way it worked in 1987 at its U.S. introduction is not the way it functions now. As the Audis did, if three wheels are without traction, the Benz will drive the one with traction while braking those without, so as to re-establish traction on them. My '05 S-500 tracks as straight as my Audis did in snow, icy patches, and in pooled water.
Because we were not permitted to use studded tires in Maryland, Virginia or DC, I cannot compare to my Colorado driving experience - where studs were permitted - on extensive ice. Neither the A4, the A6, nor the Benz was great on extensive ice in those states without studded tires. By contrast, the Audi 5000S Quattro with studs did superbly on extensive ice (I drove 55 miles each way every weekday to law school over long hills and in treacherous weather on I-25 in Colorado and never turned back with that car; but my Jeep Cherokee couldn't cut it there even with studs; it was very unstable on ice, while being good in snow).
As I said in one of my links above - while I loved my '00 S500 RWD, I truly missed the feel of AWD - I drove an '04 S430 4-Matic on a test drive one year, and it reminded me of what I was missing. Wet, dry, snowy, slushy, or icy - they're just great, and have a feel of being in control of the road that RWD lacks. So, when I came across the S500 4-Matic, I jumped on it, even knowing I was moving back to Florida within a year. The decision was vindicated on my first trip from DC to Florida, when I drove through torrential rains almost all the way down, and when a tropical storm hit while I was there. The car was just great - visibility was a far greater problem than running through pooled water 3-4 inches deep, near hydroplaning speed. That experience was validated on subsequent trips as well.
