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yes, its easy to fall into the bigger is better trap with diffuser angles and widths, believe me, I did!....... based on experience I would go for no bigger than 7 degrees, although it might look shallow and not very racey at 7 degrees, rest assured it will be fine and generate loads of downforce.......forget what you see on the back of F3 and F1 cars.......
the trick with a diffuser is to get the air to pass through it faster than the actual air speed around the car, so a good diffuser acually has a bigger entry width than the exit width, from the birds eye view it will taper towards the rear slightly.........I ran a very small diffuser on my F4 single seater and it glued the rear of the car to the floor, the exit was no wider than about 2 feet. with about a foot of flat floor pan either side of the diffuser, this is what the diffuser sucks on and pulls the rear down.......the entry was about 6 inches wider than the exit in the birds eye view, this is what acellerates the air faster than the car speed.
for your beemer, I'd take a look at the sort of diffusers that the supersports cars run, a Juno would be an ideal starting point, from memory they have about 2.5-3 feet working width, have loads of strakes, and have plenty of panelling either side, deffo a good starting point.
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