Like greasemonekey said, forget about height and try to maximize the amount of tops for your space. There are so many factors to consider, here are some that you should review and look for areas you can improve.
1) Genetics/strains/phenos. If you're growing from seed every time it's a real crap shoot. When I grow a new strain I run at least 2-4 of each, pick the best and grow those out. The strains I like I clone, then after a grow or two you know what to expect and how it responds to training. For example, this is a 6th or 7th generation clone from a Jack The Ripper strain and a specific pheno I kept. Just pulled it last night and this is the best producer I've had yet, it's been running about a year, should produce 4+ zips in a 5gal container. Typically with no training this strain wouldn't produce any more than a couple of zips. If you're in a legal area and can buy clones, you may want to try that, most clones from reputable sources should have gone through a selection process and be hardier than any single seed grow you'll try. Since you're already taking clones I'd say genetics or pheno's you're running aren't up to par for larger yields.
2) Environment, assuming you're environment is good but maybe look around at best practices for temps/humidity. Mine always seem to respond well in flowering when I'm in the 78-82F and 40-45% humidity. Look at air exchange, my plants have responded and produced better with constant air exchange and constant circulation. Even that, a small change like using an oscillating fan vs. a fixed fan has implications, fixed fans tend to dry out the plants closest to them or in direct line with the fan where an oscillating fan provides a better and more even environment.
3) Lighting. huge factor indoor, assuming since you're talking about 600w you're running hps. I switched to CMH end of last summer and my yields per watt have improved significantly, if I'm going after numbers. I just finished running a 3x3 with 5gal plants/containers and proven genetics which is looking like 12-13 zips, from a 315w CMH bulb. For comparison and circling back to my first point, I ran a small/early flip run looking for new strains/phenos which only yielded 7 zips and included a couple of runt clones that I typically would have culled.
4) Training, greasemonkey made some good points on training, the process of training itself generates more growth and stronger plants, well worth any extra time training/recovery may take.
I've gone through a bunch of tests with different container sizes/medium and agree with greasemonely's take on less/larger and well trained plants. Consider that the only time you save in a complete cycle (veg + flowering) is during the veg stage. All in all it doesn't pay to flip early, only time you should consider that is if your space is limited/maxed out.
Cheers....
