Improved climb performance is fine. And legal as well.
But you will still need to meet the requirement of a 35 foot clearance height over the threshold with one engine out. And therefore an even higher clearance over the threshold with all engines running! In no way should an airplane cross the threshold at such a low height.
The takeoff of the 747 in bournemouth may look "low", but actually it does cross the threshold at a greater height than 35 ft. It's just that the 747 is so big that it fools the observer in thinking it is too low.
The phuket takeoff photo is just crazy. and so is this one in canberra.