In the same way that there is a tension between drag and downforce on racing cars - often mentioned in F1 commentary - there is a similar tension between drag and cooling, both on racing cars and on road cars.
Creating downforce by definition creates drag. Similarly cooling the engine by definition creates drag, whether that be via the cooling system or reduction of underbonnet temperatures.
Does this not mean your underbonnet air temperature will rise---not the engine you understand since its got a direct coupled means of cooling via the "part rad"--- but the air space and with it all those sensitive little sensors/ECU'S which don't like it hot??? Surely a better solution would be to arrange additional "exits" via louvres under the wings or on the bonnet assuming they didn't screw with the aerodynamics??
Underbonnet temperatures may rise slightly, but hot air is drawn out of the engine bay from the low pressure areas, rather than pushed out from high pressure areas. If air is able to exit, then it will draw cool air in through the reduced aperature.
Something I noticed the other day on the motorway when it was wet, was that there seems to be more air passing over the bonnet, enough for the emblem to make turbulance and spray water droplets backwards.
The blind will create a larger stagnant volume which, is likely to spread up rather than down, and will deflect a little more air over the bonnet than may otherwise be the case.
I wouldn't expect the turbulence to increase significantly though, maybe even reduce. The air passing over the bonnet wouldn't accelerate more than usual, and the greater stagnation would likely smooth the radius and therefore the path of air passing over the bonnet. It would be marginal though.
Might just be more noticeable given the damp/wet conditions of late.
I love driving in the rain, because it opens up a whole new dimension is seeing your car work. You can see things you otherwise wouldn't see - marvel at the airflow around the A-pillar for example.
As 230k says, don't block the under grille because of the intercooler, but also because that's in a very high pressure, stagnant air area so works very well without creating much drag. The bonnet grille is in an area of faster moving airstream and causes more drag.
Stagnant air at the intake maximises the potential difference between inlet and outlet, and therefore maximises air flow through the engine bay. The stagnant air isn't what's causing the drag though.
The benefits here will come from reducing drag 'inside' the car, rather than 'outside' the car. Drag caused by air passing through the radiator is considerable, and generally significant around the other gubbins found in the engine bay.
but still even with cover engine is cooling down if i drive 120km/h @3000rpm 
) i alway ran the std set u,p but with a larger intercooler the viscous fan cut in quite a bit more frequently so obviously air flow was more restricted.
