I like the idea of 2 - 600's to replace 1 - 1000. If the CMH only come in 400's...well...that's that. The light spread is definately more even, but it is 20% less light. I think if it's a choice of 1 - 1000 or 2 - 400's, the 400's could make sense. BUT, given a choice of 4 - 1000's or 8 - 400's, I'd take the 1000's hands down. A single light is just not very efficient. You really benefit from multiple lights...using the spill from neighbor lights to even out the lumens striking the canopy. We run 2 rows of 5 - 1000 watt lights per row. Then I have a third row with just 2 lights. The rest of the space is for door opening, A/C's, fans, etc...So, my point is, it's important to have multiple lights. If your condition, or light choice limit you to 400's...no prob. If conditions allow, and you subscribe to the "better growing through superior firepower" technique (as I do), run the 1000's.
That being said, my partner op (who out grows me on every front) grows with 600's every 3x3 square. He also uses Magnum XXXL reflectors. It's basically total light over the whole canopy. I think he's running 32 - 600's. This is probably the best settup as far as total even light coverage balanced with intensity...it's just expensive as fuck. He started with 1000's, but a couple 20lb sales allowed him to re-design the room. I've ran a light meter under his lights and the intensity is even within 10% anywhere on the massive 300+ sq ft tray. The edges of the reflectors are almost touching eachother. I'm not sure I'd be as impressed with such an op loaded with 400's every 2.5 feet square. The 600's just seem to be a very nice size for mutiple light grows. I should also mention that the same guy has a small grow in his garage where he lives. There, he blooms with 4 - 1000 watt lights. His statement to me was that under 9 lights, use 1000's...9 lights and more, use 600's. I know the theory of what he's talking about. I'm not quite sure where he gets the 9 light cuttoff. Knowing him, though, there's a reason. I've begged him to let me take some pics of his op for RIU, but he refuses. It's the best grow op I've ever seen live or in pics.