A 4ft. 8-bulb HO gives you 40,000 lumens, same as a 400W MH. That's what I would veg with. But those lights are expensive.
A 2ft. 4-bulb would work and the price is actually decent.
The length of the veg phase is determined by harvest. As soon as I harvest I put the veg plants in regardless of size. Some are bigger than others.
If I had a bigger light I could have larger plants. My problem is with my State laws. I have to keep my plants under 12" tall and wide or they count as one of my mature plants.
The laws may be different in your State.
I would bet if you vegged under a 600W and had 4 - 1000W HPS in the flower room, you could easily double my best harvest.
For those wondering about lighting - Even if you had 4 - 1000w lights in the flower room I don't think there would be too much benefit to it (Unless you had something like 10-15 systems built with 2 posts each). The lights can only penetrate about 36" of the canopy so growing bigger plants is not feasible unless you are placing lights at weird heights to get light to the lower leaves.
Also from what I understand lumens do not necessarily accumulate. That is to say if you have two 1000w lights next to each other it is NOT the equivalent of a 2000w light. Sure if you only have one CFL and you add another it's going to help but that's not what we're talking abou there. The 1000w light simply provides more coverage and lumens than a 400w lamp however having 2 or 3 400w lamps may be better than the 1000w due to multiple light sources and the ability to provide more even coverage over a footprint as opposed to one light source. I believe the general rule of thumb is 50watts/sf of HID lighting. The added benefit of 400w lamps is they do not get as hot so they can be placed closer to the plant tops. I know you have your lamps air cooled as I have mine as well which allows us to place them even closer. Light intensity drops off exponentially as the distance increases from the tops of the plants.
These are following numbers for a 1000w HPS
1 foot away - 140,000 lumens
2 feet away - 35,000 lumens
3 feet away - 15,555 lumens
4 feet away - 9999 lumens
You can purchase a device to measure the lumens and test it out.
So my point it is just depends on the situation if adding lights will be beneficial and how they are set up, etc.