ducting the intake from the front of the case is an excellent idea as well. and you are right that at almost any speed (depending on the fan... a Yate Loon for example would be no problem) there may be a little starvation of the cpu best to keep those
fans on med to low. they just need to draft away the heated exhaust from the CPu cooler. Which of course is why your ducting example would work really well. There was a case (i can't remember exactly which one) that did something similar to great effect.
a good option as well is to remove the grills on the back on the case that gaurd the fan(s). install some wire fan grills instead to get less noise and more cfm from slower speed fans.
in the instance of the Zalman CNPS9500 or better the heatsink is designed to pretty much create the effect you are talking about regarding the ducting....... the fan blows the air from the front of the case across the fins to the back of the case. its a pretty
effective design tho rather heavy.
I haven't spent much time in the modding community in awhile.... i kinda miss this stuff.