Well I will firstly admit I haven't yet read this whole thread, I know shame shame... anyway I felt like I had to throw in my 2 cents (and sorry if anything I mention was already stated).
I feel much of this (these) questions are far to variable and I dont really see the chance of anything concrete coming out of this, BUT there is more than enough general information to make a good run for it, so here is mine...
I think there are many underlying ideas that are overlooked or misunderstood and those just add to the confusion. Like coloration for example. While it is an OK general indication of how happy and healthy something is in that environment, first priority of living things is to ensure that the individual and the species as a whole continues on. If a polyp could think, then it would chose to live, thrive, and reproduce over having the brightest colors possible. For the sake of growth rates I dont think color really has any place in this discussion because the two are seperate issues entirely (that doesn't mean I think we should refrain from talking about coloration, just that it plays a very minor/if any part in the growth of our colonies. Also all photosynthetic corals have symbiotic dynoflagellates called zooxanthellae that use light energy and convert it to chemical energy that the coral can use. There is not one type of zooxanthellae but many different species (and many many more subspecies) and each has its own strengths and weaknesses. These dynoflaggellates can be free swimming and coral can take these from the water and incorperate them into their own tissue where the zooxanthellae go to work making energy for the corals. When we see a coral bleach it has expelled the zooxanthellae from its tissue in hopes of gaining a new better suited type to replace the old. Now what this should tell us is that different colonies of zoanthus could have many different types of zoanthellae being used and thus would have different rates of growth and prefer different temps, lights, etc... Even people with the same exact "kind" of zoanthids (say Dragon Eyes) could have different zooxanthellae inside and see very different results in the same environments... I dont see there being any set way(s) of always increasing growth rates until we can better understand how exactly the polyps feed and the pros/cons of the different zooxanthellae available and how to improve conditions based on that information.
Now at the heart of this question (improved growth) there is one main universal idea to consider. The vast majority of living things have a list of "priorities" so to speak when deciding on what to expend energy on. Firstly and most importantly would be the survival of the individual (excluding a few instances when an animal neglects its own well being for the sake of its offspring, or an animal that uses the last of its energy to make one large spawning event, but these are more the exception and neither apply to this situation). If the animal is barely hanging on to life it doesn't make much sense to expend critical energy on being more colorful or growing larger for example when the polyp would just end up dying. Staying alive it top of the list and obviously there needs to be enough energy to sustain life before we can talk about growth. When the individual is healthy and thriving, then it moves on to maturing/healing... If there is damage somewhere (non life threatening) then the polyp will expend energy on repairing itself next. Finally when the individual is strong, healthy, mature, and in good condition, then it next looks to reproduction (either sexual or asexual). Following these general ideas the way to get the best growth would be having healthy thriving colonies and enough "extra" energy available to promote reproduction. The first part would require good tank conditions and nothing really bothering or injuring the zoanthids. Secondly you need as much food as possible for them (any animal really if you want good reproduction rates). Until we know how exactly the polyps feed and what they need in the way of nutrition, the best conditions for the zooxantellae, etc... I dont think we will see many concrete answers.
Much of my rambling is general biology and not specific just to zoanthus or even marine animals for that matter. We buy and trade for these animals because they interest us or they are colorful and pretty, but these animals exist because they are good at surviving and they are very good at it. To find your answers it is usually best to "think" like the animal, not like the guardian. If you want the best growth rates then either try as much as you can to see what works for YOUR tank or do the experiments and research to actually be able to answer these questions. We need alot more people experimenting and trying new things so we can actually have something concrete to offer instead of suggestions and "this is what worked for me".
Also a last little note is that if you want maximum growth rates then you need to take outter surface area into consideration. The more surface area that is able to "grow" then the more opportunity for additional polyps to be added. Here is a example to explain what I mean. If you have two identical colonies of zoanthids that are 2 inces by 2 inches square (same number of polyps). One of those you leave as a 2" x 2" square (colony A) and the other you divide up into 4 equal 1 x 1 inch squares (colonies B). Colony A has 8" of total outside growing room to expand and grow, but Colonies B has a total of 16" of outside growing room because there is alot less of the colony being "landlocked" and thus unable to really bud into new polyps. Now this does not take into consideration that Colonies B will need some time to heal and recover, but much of the time IME the increased growth area will allow better growth in the long run despite the relatively short delay for healing and recovery.
Thanks for reading my rambling, good luck to all of you in all you do, and hopefully many of us can get some new ideas and see some better growth in the future.
Rick