I didn't get to test against Show and Tell decks, but I've won a couple of tournaments with Omniscence. I think that Meddling Mage it's not the best hate because he doesn't shut off the combo (since first thing to name should be burning wish) nor slows the opponent plays, or if he does the opponent can Burning Wish for options. Moreover he dies to pyroclasm and REB, which are very likely to be seen post side.

Thalia may be better because slows the game enough for us to have enough time to kill the opponents, and Oblivion ring is an uncouterable removal for whatever is beign show and tell'ed (Oblivion is also better than Detention Sphere aganist Show And Tell because cannot be REB'ed).

However I don't like the idea of splashing a fourth color because I feel that the mana base is already weak: you need your three colors soon for Bolt/Noble/Brainstorm. Maybe a couple of Thorn of Amethyst could improve the matchup, otherwise I'd stick with more counter and a fast clock.