I had an irwini for about a year, they grow very fast from 2" up to about 7" or so, then they slow right down. and they also have a very bad habit of starving themselves if you upset them. I read about it and then mine went on a hunger strike for 2 weeks.
they grow very well on high protein pellets, I had these sinking pellets called Xtreme monster and he grew like a weed for the first few months