Don't let my setup or results discourage you. My super long duct run hurts the actual cfm being moved by my fan, not doubt about that. And I've read plenty of opinions stating it's not that much harder to cool a 400w vs. a 250w. I must admit I've always read those comments with skepticism--but heck, I don't really know. I'd just keep tweaking your setup looking for incremental improvements. Move things around, add some circulation fans, get the ballast out--I bet it will add up and you'll start getting close.

I experimented with a speed controller on my exhaust fan before I remotely located it into my attic. I found that it didn't take much airflow through the hood (WAY less than full speed) to keep the hood pretty cool. The speed controller made my fan loud as heck though. Plus I want/need as many cfm as possible to rotate fresh air into the grow area. So I run it full speed. But that might mean that if you can suck air from very close to your hood it might make a big difference.

Another option is to make a DIY cooltube as an alternative to purchasing an air cooled hood.

About the door: I do think the larger your passive intake is the closer you'll get to the actual CFM rating of your fan. But 1" by 30" (?) is 30 square inches. That's not too bad in relation to your 4" diameter fan. I bet you can figure a way to light proof it too. I'm thinking a cardboard light-trap contraption could work.

The humidifier does help bring down temps. Last time I stuck my temp probe right into the outlet of the humidifier I believe the air was colder than I was expecting. 68deg if I recall (not many cfm's though). It's basically a little swamp cooler if you're familiar with those (a common item in the desert).