[aerohead]

I'm thinking that everything they did to the car contributed to the modified performance.
That said, since aerodynamics is about the rear of a vehicle ( Hucho, 2nd-Edition, page- 119 ), and we're given that the entire rear of the Holden was in separated flow, that the C-pillar modifications, along with the 'bird bath on the boot' contributed significantly to the drag reduction.
It's my opinion though, that, since we're talking about a race car, low drag did not dictate design, and the team strove to strike some tradeoff between drag and lift ( downforce ). The rulebook from the sanctioning officials probably has some 'say' in the matter as well.
A 'blueprint' of the car would tell us a lot about what they actually did.