In the 407, I understand VNE is set by long term durability considerations related to mast bending. In the legacy 407, if your avionics had the ability to display or calculate true air speed, 140 knots true was a good wag on VNE. In the GX, the airspeed tape goes red, and the avionics ping at you like an overpeed in a jet, when you reach VNE. In rough air, at altitude, I noted airspeed fluctuations would trigger the overspeed warning.