Oh right, it was Alternate Form it's based on (now?), not Polymorph, which is MUCH better. I think they changed that in some errata.
Still not good compared to Shapechange, but ok, I guess. Look in the ELH for nice critters of really high levels. You want to take Assume Su-Ability a few times, though, since you're severely lacking behind Shapechange on this one.