Point is you will have to add some safety margin making the approach even steeper: Like in LCY it's 5.5 degrees plus 2 = 7.5 degrees that you need to be certified for. If you aim at 4 for noise abatement you'll end up close to six in real life certification. That is way steeper than many a/c can handle. There where quite a few mods done to the A318s computers, avionics, callouts, trim, flaps and speedbrakes in order to get it certified for LCY ops.