i think most race car rears are bluff ("straight") to minimise buffet, ie to provide a fixed point of separation. i think you could do a little bit of tapering, to reduce to end area. i think, from looking at F1 cars as an example, that much of the design work involves introducing the flow from under the car into the flow behind the car.
General "rule of thumb" is 7deg, but I know teams that have run as high as 11deg. I did one that was going to be very short, so I made the angle 11deg, but on a revision we made it longer (starting at the rear axle line), and sloping at 7deg, and it worked alot better.
I think too much angle causes seperation and turbulance, which kills the efficientcy. The number of strakes will make a difference too. My understanding is they help clean up the turbulant air from the rear tires. I've noticed the C6R's, running alot more strakes in the diffuser, and Prodrive always seemed to use alot.
-Dave
Everything should be designed as simple as possible, but not simpler.
There was a good article in Racecar Engineering[/b] (Magazine) last spring regarding some work that the University Of Missouri - Rolla did with their Formula SAE car. I don't have a link, but I remember reading it. If you can, check it out. They did a lot of wind tunnel testing at Ford or GM, with their car.
If your wing section has a recognized profile, you can find the stall angle in the database.
Hanley has a couple design programs for wings - check his website.
"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein