lol why do you think hps penetrates better than any other light.....because of its intensity.....and again....buds get huge under hps....because the light is physically penetrating the buds and leaves. why do you think even shaded buds grow if light couldn't penetrate through leaves. there still light brought in from the sides. why can marijauna plants grow outside even in the shade with no direct sunlight. because light travel isn't linear necessarily. justthe fact that the sun makes it sooo bright outside, even in the shade the plant is still absorbing light and photosynthesizing
I'm of the mind that penetration isn't the big deal for most tent growers as they train their plants to an even canopy using a variety of techniques and lollipop the lower stuff anyway.
If all you have is the one 630 then when you have fewer plants you are stuck using it. Wastes a lot of power and light tho the fewer plants will likely do better there is still a lot wasted.
Our power here is 23.5¢/KWH and about 90% of my cost to grow bud. I veg my plants under fluoros until they are about 8" tall before I fire up a 400MH and then only if I have enough plants to need it. I still use an 8 tube 4' T12 fixture I built 17 years ago. I wired it so I can use 2, 4, 6 or all 8 tubes but If I need all 8 then it's using 360+ watts so might as well use the 400. I see guys using a 600 for tiny seedlings and think they are either living somewhere where power is dirt cheap or free, rich as f'k or dumber than a bag of hammers.
I just checked and have my 400 only 6" away from the tops and they are doing fine. About 2' plants and the bottom growth is good too.
I have 3 - 400w hps ballasts and will use one with smaller plants then two as they get bigger where sometimes I wish I had a 600 or two but can run all 3 if needed or switch to a 1000W when I got lots going. Have a conversion bulb to get MH light off an HPS ballast if I need more MH light tho my Hortilux Super HPS has enough blue light to veg well along with the MH bulb.
I find most hobby growers like myself are on a pretty low budget so getting one 315 then adding another when they scrounge up the cash works better for many as well. Once you get a few oz in the jars it's easier to save money for more gear too.
A Light Rail® is a great addition to many gardens too and I love mine. At around $300 it cost as much as a 1000W light but can make one light do the work of two and as it only uses 9W pays for itself after one decent grow. Made in Colorado too so not some cheap Chinese knockoff that will die in a couple years.
Just my 2¢.