When i moved from perth to sydney , i drove 4000km with a 14ft and two boards on top.
In my opinion two racks and some decent tie downs sufficient, the drag thou is the biggest issue, hence the config i put the boards at, plus some duct tape to keep the windsurf board bag noses "streamlined"
I had no issues of 5 days of travelling at around 105km/h (it was the most fuel/time efficient).
In my experience you don't need to go too Overboard with transporting boards , be smart, keep an eye on movement, learn some decent knots and check everything is sweet every now and then.