Systems optimized for commercial production will get to that weight in less than 24 hours with multiple substrates. Typically, they'll grow 10 - 25 stones at once. They do this by using much higher microwave power (typically 30-60kW) at a lower frequency (915MHz) which gives a larger plasma so they can cover multiple substrates. My system is 2kW @ 2.45GHz (same as microwave ovens), so its throughput is non-commercial. But it's a nice research tool for investigating how plasma chemistry affects diamond growth. And it's fun to grow diamonds in my garden shed! :-)
In the biz environment, you typically get to something that has "good enough" economics, and you start shipping. At that point, you're afraid to touch the deposition equipment for fear of screwing up deliverables. This tends to freeze production economics at far less than what's possible.
I'm poking around to find the next 10x reduction in deposition costs. Everybody should be able to afford diamond doorknobs, don't you think?