I don't really think either of these need a buff. Nor a Nerf. The cards are almost exactly as they should be. The only change that would be made is blocking ALL offensive spells, not just Firebolt and Icebolt. They're cheap and moderately effective, except for Emerald shield. Emerald shield costs more, but then, it also blocks damage, so, as ARTHANASIOS said, "What else do you want?"
I agree that Emerald is pretty good, but still think that
all spells targeting the player, both helpful and harmful, should be reflected. Imagine the change to the meta, if a single mirror shield reflects poison - it would force mono
to diversify a little. Arsenic would still get through, but
would have to use creatures now and then, instead of just poison+SoSac. Imagine what would happen to DBH decks, if BH were reflected. In fact, think about almost any of the decks that people think are OP (other than GhostMare), which would need serious re-imagining with just this one small change.
Instead of having to nerf a variety of cards to balance out the tired, cliche archetypes we see all the time (I'm guilty of using them, too, 'cause they WORK, dammit!), just buffing Mirror Shield would have nearly the same effect.
needs some love, anyways, IMHO.