A pound is very much so doable in a 3x3 tent my friend,I've seen a grower harvest a pound in a 2x3 cab under a 400w HPS,strain was Chronic,SCRoG'd.
A simple horizontal SCRoG can easily fetch you 10-12 ounces (assuming all environmental factors are dialed,the strain is dialed,everything dialed) and even a pound,all in a 3x3 tent.
I have switched from hoods and horizontal to vertical growing.First time was 5 plants all from random bagseed,grown in FFOF in 3 gallon pots in a 3x3 tent with a vertical bare bulb 400w HPS a small fan pointed up at the light and a 170cfm inline centrifugal exhaust fan right above the bulb,sucking hot air away...I have effectively increased the amount of canopy area in my 3x3 tent by doing so.Think about it...when doing a horizontal in a 3x3 tent,you are only getting a 3x3 canopy,but by going vertical you now have four 3x3 canopies,so instead of just 9sq feet of canopy you now have 36sq feet of canopy!
First run I harvested around 12oz total (including all larf) and about 10-11oz of just prime bud...had I done better training and mono-cropped (clones off the same mom) I could easily hit 14-16 given proper genetics & training,no doubt in my mind....and with a 400w HPS to boot!
Other than the fact that you pretty much quadruple your growing area just by switching vertical in the same space,you also get little-no larf,just solid buds from bottom of plant to top!
Right now I'm working on getting a window a/c and installing because summer temps in Fla get retarded...once that's done I'll be going for my second vert grow in the 3x3 tent and hopefully do the same as the first time (with less/no larf) or better...still trying to decide which plant will do best though,it's down to two of moms from last round.
As far as doing a pound with an LED...I just don't see it happening,they just don't have enough intensity/penetration,nor decent light spread.