Mark, I do not doubt your experience and I'm 100% certain that with the car at a standstill you will feel hot air coming out from this new 'vent'. When the car is moving, however, the transition from bonnet to windscreen causes a high pressure area exactly where you're trying to vent air. The result is that air does not escape, in fact air goes into the engine bay but not enough for any appreciable cooling. For venting hot air you want to do what the OEM's do, and place reverse vents like on the Escort Cosworth. Fast moving ar tends to be at a lower pressure than that of a slow moving air (these are the principles that keep aeroplanes up) so when the air goes over these types of vent the slow moving air in the engine bay is literally sucked out and the differece is huge.
All said and done, bonnet raises look as though they should work, but due to the location, they don't.