I know that's why I said it but I reckon a three to four tree width strip will be enough to make a real difference.
And the ones I was thinking of was those ones that everyone complains about when next door puts them in, they grown over a meter a year and the foliage is thick as hell all year round.
Go borrow a Chinook and park it behind some hedges and run around with a noise meter. I am sure your boss must have some that she could let you try it out on.
Oh and if someone is reading this and thinking h'mm might work remember to horseshoe the ends of the wall in otherwise the sound will just diffract round the ends and act as two point sources at the ends. Ideally have an section which goes past the bits that curls in to catch the spillage.