Figure also that a watering system will go below the plants. A flood system tray is about 2-2.5' tall to provide room beneath for a res tank.

You really don't want a plant much more than about 3' (1m) tall. That's about the depth limit for foliar penetration of a 1000HPS.
LOL - yeah, so that 9' headroom gets eaten up real quick. However, with the amount of space I have and the limits on the number of plants, I think larger plants are the way to go. If I have to devise some supplemental side lighting, so be it.

12 in flower still is too low a plant count to consider using SoG in the standard fashion. I put twice that many clones into flower in my SoG every 2 weeks. Plants grown in true SoG style only yield about 3/4-1oz per copy, but if you're harvesting 23 every 2 weeks as I do, you make up for the low per-plant yield a bit.
So, figuring an eight-week growing cycle, and two weeks veg, you've got what, five trays? Obviously you have a lot more space than I. I'll have essentially 10'x10' for flowering. I'm figuring approx 2/3 of that space for plants finishing their flowering and 1/3 for plants starting their flowering. That should give me a harvest about every four weeks.

I thought I read somewhere that California has a sog provision in the law that goes by the total square footage rather than number of plants. (It was like 6 plants OR 99sq.ft. in a sea of green.) Anyway, I need to check into that.