- Fuel is 100-200 Kg more than minimum; anyway, not enough to reach the destination if flying with flaps extended at low flight levels.
How did he come up with the fact you can make it above 20, 000' as you stated the FMC doesn't account for flap extended like it doesn't account for gear extended, FCOM 2 would have no perf figures for flap above 20, 000'. You can't assume Fuel flow and ground speed would be better.
What will happen when suddenly you hit the sweet spot and flutter rips your asymmetric flap off or your outboard aileron etc.
Is your colleague a test pilot?