I guess you mean connecting a tube from the case exterior to the heat sink fan. it seems from articles I've read that 4-5 C can be saved using this technique. instead of tying up a big ol' tube inside the case, I just run my computer without a side panel. I save 4-5 C like that as well. Regardless of how well air flow is set up, the inside of a case will always be warmer than the outside, unless 1) it is open to the outside environment or 2) air is piped into the sytem .
I also think the ducting a fan to the outside will cause the fan to undergo more stress/work, since it is pulling air from a single, uniform direction over a given length.