747 Focal,
I used to fly 747-100/200 with both PW and RR engines until a couple of years ago, and I have a different understanding to you. We used flap 20 for normal take-offs but flap 10 for noise abatement (with climb power set at 1500 ft agl) as, at V2 + 10 kts for the appropriate flap setting, flap 10 gave a steeper climb gradient after lift-off and thus reduced the noise footprint. HOWEVER, we had a special noise abatement profile for LHR and LGW to beat the close in noise monitors. This was a flap 20 take-off with climb power set at 1000 ft agl. These meant that we had climb power set before the monitors and, although we were lower over them than on a "normal" flap 10 noise abatement profile, the dBs were lower as with the flap 10 profile we still has TO power set. alatriste, you were on the right lines.
Genghis,
You need to appreciate that the philosophy behind rotate speed and initial climb speeds in light aircraft is different to "Perf A" aircraft. In, say, a 747 you are no where near Cl max during rotate. In fact, Cl max/Vstall are not factored into Vr. It is all related to achieving minimum climb gradients (i.e. V2, although this can be capped by Vstall) rather than the maximum gradients for light aircraft, where Vstall does become important for rotate. Do not confuse different take-off performance categories.