Sonny,
I think it depends on the wattage and amount of bulbs you are using. I can imagine that the same fan cooling a Sunpower 4x24 watt requires a lower voltage then the Sunpower 4x80 watt

Also, the 6 bulb version has two fans, so actually one fan per 3 bulbs instead of four. That also effects the cooling abilities.
The PAR results can only be applied to the tested ATI pendants. Besided that the test was performed using a Powermodule which has MUCH better cooling.
I would suggest to let the fans run 15 minutes after all the lights are off to cool the ballasts, like stated before.
To get the optimum output without the ability of testing with a PAR meter, try to aim for the ideal temperature for T5 HO bulbs (35 C)
I measured the air when entering the bulb ends, and at the end of the pendant. When you take the average and get around 35 C I think that's the best setting.
Said that I did some tests myself and my ATI Sunpower 4x39 pendants (one fan) gets 31 C at the bulb ends and 55 C at the end of the pendant at 12 V.
So the cooling is insufficient on the Sunpowers to say at least. I'm thinking about modding the Sunpowers with larger, high quality fans.
I suggest to run at 12 V Sonny, and ignore the sound
Leonardo