I will repost this diagram from before the crash. It is from tests done in the VW wind tunnel - a good few years ago, judging by the model of VW Passat used.
The first case has a Cd of 0.53 but at 50mph produces 76lb less hitch
weight than when it was stationary - this might be 1/3rd of the typically small European hitch
weight.
In case 2, with a roof spoiler, the Cd drops to 0.45 (15% less drag), but still produces a 71lb reduction in hitch
weight at 50mph.
Case 3, with a sloping trailer front and no spoiler, has the same 0.53 drag coefficient as the first case, but has no loss of hitch weight at speed.
So VW reckon spoilers work.
Andrew