I've done something similar to this a number of times, in different size tents.
Placing a dehumidifier in there wouldn't be my first option. Having your dehumidifier outside your tent will give you the same result by dehumidifying the air being exhanged into the room, without it taking up floor space in the tent, and without dumping heat into the tent.
A 1000W bulb in a 5x5 will cause your RH to blow through the roof, and then the dehumidifier will try to dehumidify, only creating a vicious cycle of heat/transpiration/RH rise/dehumidifty + Heat, and then it repeats. My personal opinion would simply remove the dehumidifier from inside the tent and place it outside. If you have your exhaust on, it will be taking the newly dehumidified air and dumping it out of the tent anyway making it less than optimal - very little dehumidified air will stay in the tent at that point (it may make it's way back when your room cycles enough air, but still no reason to have it inside taking up valuable floor space).
I personally just like to cycle the air from the entire room - and then you can easily control the rooms humidity with your dehumidifier, and cycle that air into the tent. I've done 8x8s with 3000w. The easiest way to keep humidity and temps in check was simple air exhange on something like a HUM-1 from C.A.P. or some other environmental controller. I've never had a dehumidifier in my tent, but always have one in the room it's in!
I think the filter/fan above your bulb is a great idea for air exchange, and to pull the heat from the bulb. If you're not using sunpulse UV bulbs, you could hook up a cool tube over your bulb, right before the fan so you could have your plants get closer to the bulb and not worry about them getting burnt (5x5 can get real small, real quick if you over-veg). If you are using Sunpulse though, it is recommended to not use glass around the bulb if you want maximum UV output.
However, sucking the air "right above the bulb" or sucking the air from the top of the tent really won't make that much of a difference as long as you're moving a good amount of air. Any 6"+ exhaust fan will move PLENTY of air to keep that temperature stable anyhow.