Like signman says measure temps at the tops of the plants (or where they're going to be in your case, 4" is about right as you're using cfl's)

If i'm understanding you correctly you're using 2 fans, one for getting the heat out of the cab and one for getting fresh air into the cab? If thats the case, i'd use both fans for drawing the hot air out and make some more holes to be used as passive intake. This way you'll double the amount of air circulation and i'm willing to bet you'll solve your temp problems

e2a you might be able to get away with just adding a passive intake by adding another 4" hole at the bottom