Well, Im not sure it would felt as well. Part of the magic behind felting is the extra room for friction that is allowed by those loose sts on big needles!
You may want to knit up a 4x4 swatch on your 9's and test-felt it. Anything is worth a try!!
Also, you may want to increase all the pattern numbers by 25% or so....its gonna turn out smaller and the booga is not exactly a big bag to begin with!