igo: that's a tough call. The width of it suggests to me it's making things worse, not better.... BUT the height and depth look reasonable.
The best way to tell for sure would be to tuft test the trunk with it removed, to see if there's any flow attached at the back of the deck lid.
Like this...
Video: tuft testing (rear) 1993 Ford Mustang notchback
What angle do you get if you follow Phil's advice and draw a line from the trailing edge of the roof to the trailing edge of the trunk (a) without, and (b) with the spoiler?