Alright from personal expierence, and research. I have found that if you grow a larger amount of smaller plants you could achieve a larger yield. If I were you, I would seriously consider doing a SOG grow. If you do not know what that is, google it. It's easy to find out, and easy to do. 18 oz's on your setup is possible. But I would suggest not getting your hopes up as it is your first grow...
The best possible advice you could get from anyone at any point in time is this: Start browsing these forums dog!
Before I started growing, I learned all I needed to, then read the "plant problems" forum for weeks just so that if I EVER came across a problem that so many people do, I woul dknow what to do. I've grown around 15 plants, and have been growing since the day I signed up on this forum. (can't remember when that was exactle, only bout 6 month's tho) and I've had GREAT success with growing! And the ONLY reason, seriously, is because I've read this forum for countless hours. So the best advice anyone could give you... Is to start reading!
Also I purchased the 'cannabis grow bible' incredible book! If you can afford it, I'd strongly suggest it!
Just remember, if you really want that yield your hoping for, your gonna need to SOG (Sea Of Green) it! IMHO
you wont get 18 zips off of one 600. i vegged for 8 weeks and wont get that. and had high yielding strains.
I really REALLY don't think he's going to do so either. It is possible though! If you think about it, a gram per watt is what the BEST of the BEST go for correct? Well a gram per watt off a 600W HPS, would be 21.43 OZ's. I would be willing to bet my entire bank account that he's NOT gonna get this, BUT it is possible... Just saying...