Grow coverage area on larger hoods?

Here are two examples

Quantum Massive
http://www.ehydroponics.com/quantum-massive-8reflector?&cid=12
331⁄4” x 20 5⁄8” x 8 5⁄8”

XXXTreme 8
http://www.ehydroponics.com/xxxtreme-8inch-hood.html?&cid=12
8" measures 43” x 31.5” x 17”

My question is, when using a 600 Watt bulb with these, how much does it improve the coverage area? Doesn't have to be those two examples, linked them simply just to give examples. Hopefully someone on here that has XL hoods can give me some insight. Getting ready to upgrade and want to know whats the best coverage area i can get with a big ass hood.

If i got a big enough hood, could I put an 4 x 8 tray under it and be able to cover most of it?
Thanks

4x 8 you should use 2 600's one is usually good for 3x 3. I use a 600 in my 5'x 3' box

I'd put 3 600's over the table. I have 3 1Ks on a 4x8 w/good coverage. Remember, the closer to the tops the lights are the smaller the footprint. I keep the 1Ks about 18 to 20 in (max) above the tops.