So in all honesty I would need 3 A51 190's to hit an elbow. They would pay themselves off in one cycle. Just thinking out loud here. I am down with investing in the future and when has the initial amount in mind ever been less then the reality. Never if you ask me. I threw $1k out there to my significant other and she winced, but LED was her idea to begin with. It's not like we don't have it or it would affect our lifestyle or anything, it's just more then I was planning on. Wrapping my mind around it and I should present a good argument to my woman.
1 unit from 2 a51 is doable, but that's not 2g/w(1g/w with your two 400hps). The new a51's have a better chance than the older, but no one has ran a cycle yet they are still too new.
I don't know if you looked up dawg...he has a few a51's and pulls nicely. If Dawg would get some good clone genetics he would hit a unit with 2 SGS's. He grows from bean and bad pheno's keep him back. He get's over 8oz with one and th other got like 5oz...but had they both been clones of the good one he would have had it no question. And that was in a 2x4 tent. So I know it can be done.
It will pay back pretty quick and if you count your harvest towards that, super quick. Girls are smart, and they see things more long run sometimes. Long ruins the name of the game with led's and savings.
2 XGS's will do you great to start. And may be plenty when all is said and done. I was just telling you what the past/present has shown.