Been doing some testing of Young Pyromancer in a Grixis Tempo shell with Phazonmuant, here's where we're at so far. I've been very impressed with the card.
4 Delver of Secrets
4 Dark Confidant
4 Young Pyromancer
4 Brainstorm
4 Ponder
4 Gitaxian Probe
4 Daze
4 Force of Will
3 Cabal Therapy
2 Thoughtseize
4 Lightning Bolt
1 Forked Bolt
8 blue fetch
3 Underground Sea
3 Volcanic Island
4 Wasteland
Numbers may not be right, stole a lot of design from Canadian Threshold, but we've played games vs TES and Miracles to get to where it is now. Some notes on this - often you don't have a turn one play, which makes me think something is misbuilt. Do we need to try to fit Deathrite in here? Grim Lavamancer? Pierce / Snare / Stifle? Definitely save your Gitaxian Probes when possible to maximize Therapy and Young Pyromancer.
Quick try at the sideboard, not fully undertanding what matchups are bad:
1 Cabal Therapy
2 Flusterstorm
2 Red Elemental Blast
2 Perish
3 Surgical Extraction
1 Engineered Explosives
2 Sulfuric Vortex
1 Sulfur Elemental
1 Rakdos Charm