I really thought that diffuser was going to make a difference. It looks exactly as I pictured it in my mind. You may be right that the angle is too much and is causing separation.. I have my doubts that that's the issue though because from looking at some other examples of diffusers, your angle, although sharp, doesn't seem too extreme.
Any chance there's a pocket of air collecting under it and air pressure keeping flow away from it? Most race cars have a completely flat bottom that does not allow for this. Maybe if you extend the plate forward more to keep that from happening?
I would maybe try a combination of both...
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