Okay,thanks!
From fluid mechanics,any diverging nozzle should never have a wall angle exceeding 7-to-8-degrees,or separation/turbulence will occur.I suffered from this at Bonneville by mimicing a Mustang GT diffuser (which I had to close off completely with cardboard and duct tape).
The 'trumpet' is a good technology though if tuned in a tunnel.
Recently,the Bochum University team used it on their WSC car
In 1978,Morelli used it on his CNR concept,creating wheel fairings to enclose the diffuser,helping to achieve zero-lift.
The 2nd-gen Prius has a fine 'trumpet'.