Your size room only needs 108 CFM for a air exchange of 1 time per minute. MUCH MORE than enough as a grow room only needs an exchange rate of 3-5 times a minute. Not sure what the person who told you you don't have enough CFM is talking about. You're suppose to add 20% for each 1000watt light IF they're not separately vented, so if that's true than you STILL have enough CFM's as 2-400watts would only add an extra 40CFM needed, you're still over that total of 148CFM with the 200CFM. Add on carbon filter that's another 20% you need to add. So that's now 188 CFM's needed, which with your 200 you are STILL above. You have a PERFECT room.
Sounds like you need to remove ballasts from room to lower heat, even when kept high in room the heat from them up high can slow up the exchange of hot air out, and I believe one 400watt would be fine with a passive, but you may need to force intake in with the extra 400watt. You could try to just make the passive larger, at least 4 times the size of the exhaust. I had a wood framed room lined with panda and for my veg side I just fit in a basic furnace filter, about 15"x12", into the wood frame and secured panda film around it. With a 180CFM exhaust in a 3'6"x4'6"x7' high room it keep temps at 74-78 degrees 24/7 cycle with a digital 400watt.
