You have answered your own question here. "Provided they got the room to grow, get light, and air..."
No didnt answer the question friend, because if you go back and read, the question is 8"x8" to tight? meaning will they have those three elements in that area. see how that works, coloring books can be fun!
You will not have these elements in the conditions you describe. Whether you want to believe or imagine it or not.
Well.......... I guess, I cant imagine that!
And I am sure it won't. 40" x 40" equals just over 11 square feet. And while a 400 watt hps can put out an initial 50,000 lumens, it is still insufficient to light your space (research "inverse square law"). I am speaking from first hand experience, not conjecture or supposition. You will simply not be able to get your light close enough to provide adequate lumens for maximum growth. Again, I am not imagining this- it is from first hand experience.
Maybe if you did imagine it, you would have achieved
Thats great about the inverse law stuff, you should build rockets! 40" is a whole 2" more a side for the highly sugggested 3x3 per 400 w, did I say an extra 2" per side? I bet that the plants on those outside 2 inchs are going to suffer because on inverse sq law
I attempted to do what you are doing with some differences. I did it in 10 square feet under
2-400 watt bulbs with 10 clones that were initially about 8" when cut (larger than yours, but not really in footprint). They rooted in 10 days. I vegged them for a little over 2 weeks. This pic is at about 4 weeks flower and is just under 1 square foot in space- I could not have fit any more plants in my space and still have allowed for effective air flow. Passive air intake at the bottom, ducted (outside grow room) fan driven intake at canopy. 2 exhausts, 1 at canopy, 1 at lights. Final harvest yield was under .4 gram per watt. Not my best effort but about as much as I could hope for under the conditions provided. More plants would not have yielded any more final weight.
Maybe you didnt have the weight, because they vegged up to thick, before flower, due to the extra lumens? Did you prune them at all?
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Bottom line Devilfrog- you can absolutely fit 25 plants in 11 square feet under a 400 watt light. And that sounds like that is exactly what you are going to do. So just do it.
Ok will do!
The membership here, while we all may do a little bragging now and then, really has no other motivation than to try and help by sharing our experiences. I will not try to offer you advice. Some very talented, respected, and experienced growers have already offered you suggestions and insight.
Do your grow, post your results, and prove them wrong. Good luck to you.