Little to no air will flow through the existing radiator setup if it has a traditional core.The air must change direction to flow through the tilted radiator core. Your going to need good baffling to force it through. There will be added drag from redirecting the airflow.
FYI, the older corvettes had radiators tilted to a significant degree but they also had special cores that were arranged to match the tilt (i.e., you can see straight through them when tilted to 45 degrees or so).
A few degrees are okay but the greater the angle to the airflow, the greater the flow reduction.
MiataV8, I may be wrong, but I learned that the low pressure (exhaust) side of a radiator is the driving force of the airflow through the radiator, and that the air is more sucked through rather than pushed. The amount of air flowing through the radiator is relative to the strength of the low pressure zone behind the radiator. Otherwise, take for example a normal car who's engine bay has been sealed, the pressure would be higher behind the radiator than in front, and there would be zero air flow or possibly reverse flow. This is true in aircraft anyways.
Usually there is some form of an expansion chamber before the actual radiator face to allow the air to slow down and expand. Of course the length of the chamber is relative, but that is why the opening of the chamber is usually narrower than the radiator. This is also why the amount of drag attributable to the angle of the radiator would be negligible, since the air has slowed down considerably before going through the fins.