Here's the sideboard I've been tinkering with on MWS (my maindeck is pretty much k^j's):

4 Leyline of Lifeforce- Amazing against Counterbalance/Chalice. I usually side out all four Glimpses and just try to beat down.

4 Thorn of Amethyst- You can outrace gy decks, so I decided to shore up the combo matchup because they're faster. You can drop this T2 rather easily and then try to race them while they scramble for Chain of Vapor.

3 Fecundity- Easily my favorite card in the sideboard. Lessons the blow from non-Swords removal mightily. Allows you to chump block Tarmogoyf all day while cantripping into combo pieces. Obviously good against Firespout. You can even cycle 1/1's against Engineered Plague until you find a lord.

2 Gleeful Sabotage- 2 CMC and better potential pushes this above Krosan Grip's consistency, although I can easily see why one would favor Grip.

1 Elvish Champion- MVP against U/G based tempo decks like New Horizons. Goes well with the beatdown plan against Counterbalance. Pact target.

1 Nullmage Shepherd- The best artifact/enchantment destruction Elf available in my mind since her ability is recursive and isn't too much harder to activate than Shaman/Zealot. Pact target.