Some time ago I made some measurements comparing the air speed behind the spinner of Tru-Turn spinners with their lightened backplate and with their Turbo-Cool spinner which has fan shaped spokes on the backplate and openings in the cone. http://www.truturn.com/cgi-bin/store...30&maxp=30

I chose to measure airspeed, as motor temps are very dependant upon air ducting. Measurements were taken in flight using a pitot tube mounted 1/4 inch behind the spinner at a midpoint between the shaft and outer diameter.

Results with no spinner, a standard spinner / solid backplate, a standard cone / lightened backplate / and the fan backplate - vented cone indicate the most flow speed with no spinner (comparison point for others) and no flow with the solid backplate / cone.

The lightened backplate and solid cone resulted in airspeed of 35% that observed with no spinner. (This cone had larger than necessary prop cut-outs.)

The fan backplate / vented cone provided 60% of the airspeed provided with no spinner (and looks way better). 

My conclusion is that a spinner with an internal fan and cone openings provides significantly more airflow than other systems. The cooling benefit of the extra airflow is highly dependant upon how the air flows through / over the motor.