Ok, I've read your thread. Your 6" inline 267 cfm fan should suffice for what you want to accomplish. You did however state that you are using 4" lines. Well, I am saying you should use 6" a line. I knew you weren't gonna like that. If you cannot use 6" lines throughout, can you do the majority of your line 6"? I don't know what kind of pipe you are using. If you're using coiled flexible dryer vent line, get rid of it. You need to make the line as 'smooth' as you can to get the best possible airflow. The flexible lines, especially for a long run such as yours is going to hinder airflow due to the ribs (coils) inside where the air is supposed to flow. One of the answers you had on your thread is to have as few turns and bends as possible. If you do have a bend let it be as gentle as possible. The smoother your pipe is on the inside, the better your airflow. That's why an airplane has a smooth surface....and an airplane wing without rivets protruding on the surface will fly better/faster.

Going back to the size of your line. You should be using 6" warm air pipe. If there is an area where this is impossible, then use the 4" where you have to, then using a 6"to 4" reducer, continue with the 6". Please be aware however, whenever you do, there is a lot more pressure in the area of the 4", and it will produce turbulence which is not good for airflow -- similarily if you run a 90 degree elbow.... your are hindering airflow again.
Your fan should PUSH, not pull.
I have the same fan you have, using 6" lines (mainly 6" warm air pipe) and only using flexible line for maybe 2 ft on each of THREE 1000 watts lights, and I have total of about 20 feet from start to finish.....no problem.

You haven't mentioned anything about the internal ventilation of your room. oh, yea, you're using CO2.... that could be another problem without A/C . No matter what size fan you have to get rid of the heat from under the lights, a 600 watt bulb puts out 2000 btu's of heat, you will never get 100% of those btu's removed with the fan. After your light has been on for a couple of hours, touch your hood. I'll bet it's hotter than the ambient air inside your room. Eventually that will heat up your room just as if you have a frying pan heating up in there.
SpaceNeedle