The only notchback that I know of with that sort of rear window angle / trunk lid height that was modified for attached flow is the VL Walkinshaw Commodore - unheard of outside of Australia.
This pic shows how much they had to raise the boot / trunk lid to get attached flow... and a spoiler that really did something.
Others to think about include the deflector on the BMW 'Batmobile' 3.0csi...
...and the Impreza WRX (standard STi?) roof deflector.
Some AirTab vortex generators on the uppermost part of the glass / trailing edge of roof may improve flow attachment on the boot / trunk spoiler -
may...