Another thing that might help is putting a little tower fan($20) right in front of the intake to suck air in and force the cool air up to force the hot air away fro the plants. In my set up I have a regular old bathroom exhaust fan to suck air out and started with 4 4" air intakes. The temp was too high so I cut 7 more intakes and the temp didn't change. The problem was that the air needed to be circulating around the room and a little tower fan in the corner droped my temp by 10 degrees.

If I were you I would put the new exit in with a fan to suck out the air. I read somewhere that every 90 degree turnin tubing causes you to lose 1/4 air flow, so you are running at half with the 2 turns compared to a strait tube. If it is flexible maybe straiten it out so it runs at an angle from fan to exhaust with as few turns as possible.