Bizjet pilot, While I agree it must be nice not seeing the departure end of a runway, it is normal for an aircraft with a MTOW of 394,625KG (B747-400). As for the risk of a derate take-off, an engine failure at V1 is designed into the formula for all take-off data. If you were to increase thrust you stand the chance of loosing directional control (some take-offs are VMCG limited).
I found a couple of interesting lines in our manuals. I stated earlier that we refrain from using 20% derate but I find that we can use the 20% derate when operating with low weight and aft CofG take-off's. Also, we have a line in the books that states the Maximum thrust reduction authorised by the FAA is 25% below any certified rating.