If you stack the minuses for firestorm and rapid shot, it's a -4 or all the shots... maybe a -2 if you manage to crawl up that entire chain (manyshot is a waste of a feat anyway (for bending... and well even archery...) Personally, I think rapid shot is a viable feat, but you have so little feats to play with as a bender in the first place, so things that you guarantee get are the skill focus feats, a few of the bending specific feats, so rapid shot becoming a hard choice for a feat is a good thing.