I know the question is about 33 rear spoilers, but the 1994 155 BTCC spoiler controversy Kev talks about above, gives you an answer, see below (coutesy of Touring Car Times to save me typing
"The Alfa 155 Silverstone version featured adjustable front and rear spoilers. The road cars were sold with the front spoiler and rear wing retracted. But with a special kit, that was supplied with the cars, the rear wing could be raised a number of centimetres and the front spoiler could be extended forward with supplied rivets. The aerodynamic advantage for the road cars were insignificant, but for the race cars the advantage was very significant.
As Alfa Romeo won the first five races, the other BTCC manufacturers were quick to protest. The controversy culminated during the seventh race of the season at Oulton Park. Alfa Romeo was told by the BTCC organisers to run with the spoilers retracted. But the Italian manufacturer had none of it, claimed that their cars were within the rules and left the track in protest.
The rules were quickly changed: "...manufacturers must indicate the entirely completed position of any adjustable aerodynamic bodywork and will only be allowed to run in this entirely completed position."
Alfa Romeo accepted the rule change and returned to the following round with the rear spoiler retracted but the front spoiler still extended. While the others got closer to the Alfa's after the rule change, Tarquini kept winning races and had a big point gap. Tarquini won the title in style."
In short the rear wing has little effect, but a well set up front splitter has a significant effect, especially on a front wheel drive car.
So get to work on your 33's Veloce splitter
1994 was great year to be an Alfa fan, certainly got Ford and Vauxhall's attenion. Sadly 1995 was a bit of a dissappointment once every one else got splitters and spoilers.