EDIT: Remodeling the Car On hold until more progress This is an Ibishu Covet Based on the Ford Sierra Cosworth, I Call It the Ibishu Sierra Covet or ISC for short Progress:
Sorry to burst your bubble, but I would not make a sierra styled version out of the ibishu. The spoiler looks like broken, it would just add some weight to the car
I can try to explain you: this spoiler is useless. It won`t influence to aerodynamics, for this it should be placed on a roof. And my suggestion: hood/roof scoop will be good here.
Or, redo the spoiler to where the flat edge is in the same straight line as the roof, if possible, angle it downward to a point where it'll actually create any downforce. But even with the Covet a spoiler like this just wouldn't work, considering how steep the hatch is, the Covet wasn't designed for something like this. If anything, a smaller version would work, but overall, if you wanted to do a Cosworth, I wouldn't recommend the Covet for such a thing.
If, someone's created a coupe version, yes. But for the sedan, really shouldn't be much needed done, since the '88 Pessima already comes with a spoiler and the sedan version of the Sierra doesn't have the spoiler the coupe does, since it's not a hatchback. The only difference to make the Pessima a "Cosworth" is make it 4WD and engine mods/tuning. And the SBR4's bodystyle seems the closest, but it's too new and modern for something like this.