I haven't heard about henna stimulating growth. But it does strengthen hair, so it may help retain growth. Some of the ladies on the henna board said that it helps to split-proof their hair.
100% pure henna is supposed to be good for color-treated hair. But you should always do a strand test, and read up in the henna forum first. But it might me a good idea to start with cassia. Depending on your hair, it can take either one or two cassias to see results.