Mist's magic cap in FE9 is 26 with an average of 25 (which means it's not too unlikely that she'll hit it). You're probably thinking of Rhys's magic cap with that 29.
Ah yes, so it is. Her resistance cap is 29, that's what I was thinking.
But even with her decent strength growth, she still starts with a horrific base of 1, which means that fighting isn't exactly going to be one of her strong points until near the end of the game
Hence Sonic Sword in chapter 18

. I guess its just my playing style, but I always go out to train Mist, and she always freaking owns for me (maybe I'm just lucky). The only time I ever use the Hamerne is on the sonic sword. Usually I can have her take out the entire right flank of the last level without much problem. I do play favorites though, since I don't really ever need bonus experience I usually promote her to a Valkarie as soon as possible so that I can get her sword level up. Plus I do usually use the Arms Scrolls on her. Its not like Nephenee or Rolf really need them, since they have two class changes to get up to S in their ONE weapon type, and Nephenee doesn't even need to since you never get the Wishblade.