8000 lumens isn't terribly bright. I think I've seen some fluoros at 6000. If you're prepared to put 200W worth of power at it, could you go a 175MH (~15,000 lumens) or a 250 HPS (28,000 lumens)? Better to go with a single bright light than a pair of comparatively dimmer ones.
You're getting pretty much the same as I am. I pull about 23oz every 2 weeks off 23 SoG style plants. However, if you're flowering only with a single 600, you are kicking my ass from here to Mars and back! I run a pair of 1000s, as you know. If I were you, I don't think I'd be bitching too loud.
Mums are a piece of piss to keep. No sweat.
You might rethink your layout. You won't need near as much space nor light for vegging if you switch to SoG. A 400 will support 8-10 mums, might be able to do it with a 250HPS. About 5% of your floorspace can go to vegging mums, the rest to flowering.
Nah, the 600HPS is a very fine light. Use 'em. I could easily see my op with a 600 over each tray. Without cooltubes, a 600 can be spaced closer to leaves than a 1000. A 600 usually does about 95,000 lumens, a 1000 does 160,000, but since a 600 can be dropped down lower, the actual lumens per sq metre (lux) applied can be the same or higher than a 1000.
However, when fitted in cooltubes, the 1000 can be spaced the same as a 600- and beats its pants off in the process in terms of laying down lux on the leaves.
The downside to using 600s is that they are marginally less efficient in lumens per watt than 1000s and you have to have twice the number of ballasts and reflectors. It'd work fine, let 'er rip.
Sure, you could use coir, could use pellets, etc. Whatever you can stuff in a pot. I'm not a fan of coir and have seen it break up when exposed to H2O2, but a knee-hi stocking over the bottom of each pot should contain any loose frags that might foul a water pump. However, it will work for you.
Never- and it's not because I'm real smart or real lucky. It's because I'm very security minded and I always plan ahead. I calculate risks and evaluate security at every turn. It's a lot of mental runtime that I'd rather not do, but it's part of the gig.