The only thing I've ever found the spoiler useful for is blocking out rear headlights at night.
Unless you're running at high speeds it prbably makes very little difference whether you have it on 1 or 2. Also, getting too much rear downforce could be as bad as not enough, depending on what type of driving you're doing.
Perhaps we should borrow a wind tunnel for the day?
I've played with these Halford like items for a while.......
I've only noticed any difference once your well into 140mph plus at places like Ford bend at Goodwood. The thing is with the weight the car has the spoiler is hard pushed to make a difference to the "Handling". From raked to not raked it does not make any lap time difference, Remember speed is generally in the mind, so, what it does do is perhaps build some cycological confidence on really fast tracks!
What it is really there for is to reduce lift (which it does a great job of doing) and at the higher setting will make high speed lane changes more secure. So downforce is not really in the equation but reducing the effects of the air under the car (like a counter balance) it is good for.
On a 34 if you really want to feel a difference take of the rear diffuser! It makes a huge difference to the Top end acceleration and makes the back alot lighter. Glen know's and may wish to ad to this thread?