Problem with putting the fan before the cooltube and then running ducting out of the tent is that the fan blowing over the bulb and into the ducting creates positive pressure. Basically it creates a column of air in the ducting ahead of it that is a higher pressure than normal, and because the fan is pushing against it,it actually slows the fan down reducing it's efficiency.
Hey Harrekin. You're right in that pushing air into the duct slows the fan down slightly, but that doesn't in itself reduce efficiency. The higher pressure means higher air density, so each sweep of the fan blades pushes more air. Whether pushing a column of high pressure air, or pulling a column of low pressure air (which is just as much work) the overall flow through the duct will be virtually identical. An extreme example of how higher fan speed does not make for higher efficiency is blocking the intake on a hoover - the fan speeds up to a whine due to low air pressure inside, but little or no air is moving through the hose and so efficiency is next to zero.
Pushing into the duct will slightly reduce fan RPM, but this is good as it reduces wear on the fan bearings and reduces noise without impacting overall airflow through the duct. In addition, as the air is cooler before it passes over the light it is denser, so each sweep of the fan blades pushes more air, further reducing RPM compared to a pull fan without reducing efficiency. All in all though, at the relatively low temperatures and pressures involved in a cooling duct on a grow light, the efficiency differences between pushing or pulling are too small to worry about. So long as the duct has a large enough diameter the overall efficiency will be almost identical. On average though a fan will last a little bit longer if pushing cool air than pulling warm air.
Splifchris - How did your friend get busted if I may ask? What led the blue gentlemen to his door?