Hi
flyburg,
The world of runway friction and how this is accounted for in takeoff and landing performance is a difficult subject which we have been studying many years now. Al lot of the performance stuff in the FCOM is advisory and its content depends on the origin of the operator and aircraft manufacture. Differences can therefore exist amongst the operators using the same aircraft. Note that the risks related to operating on contaminated runways are much higher than on dry runways.
Runway friction measurements on contaminated runways can be misleading and can give a wrong impression of what the true braking friction of the aircraft will be. There have been numerous initiatives to improve the correlation of these friction devices used by the airports and the braking friction of an aircraft. None of them gave us the best solution. The latest initiative is called TALPA ARC which to me seems to bring us closer to a better solution.
Also take a look at these publications:
http://www.nlr-atsi.com/eCache/ATS/14/919.pdf (Part Runway condition pages 23-25, and Section 5.2.4)
And
http://www.nlr.nl/smartsite.dws?id=4381