Growth rate is partly a reflection of environmental conditions, and partly genetics.
5 to 6 feet a year is common, but the overall wood quality is poor. Similarly, there is much literature on the affects of improving environmental conditions and the increased growth rates resulting. But, merely by improving growth conditions it does not follow that there is an infinite increase in growth rates. All biological organisms have limiting factors, and all reach a plateau beyond which it is difficult to proceed further. Each tree species has its own inherent growth rate and growth requirements such as ph, moisture, shade, rooting needs and so on.