Warlock class card drawbacks are a little steep. I suggest a new class mechanic for Warlock cards.

"Shuffle a random card back into your deck."

This mechanic would replace many of the drawbacks on cards that are too negative. Since this penalty is not as harsh as discarding cards, current cards with that mechanic will need to be adjusted slightly. The hero ability allows the Warlock to draw them back out at the cost of life and mana. I see the following changes to make this mechanic and card cycling of the Warlock work:

Sacrificial Pact: Give a Demon "Deathrattle: Restore 5 Health to your hero." 0 Mana
Soul Fire: "Deal 4 Damage, Shuffle a random card back into your deck." 1 Mana
Corruption: unchanged
Mortal Coil: unchanged
Power Overwhelming: unchanged
Blood Imp: unchanged
Flame Imp: unchanged
Voidwalker: unchanged
Demonfire: unchanged
Succubus: 4 Attack, 2 Life, "Stealth, Shuffle a random card back into your deck." 2 Mana
Drain Life: unchanged
Sense Demons: unchanged
Shadow Bolt: unchanged
Felguard: 4 Attack, 4 Life, "Taunt, Shuffle a random card back into your deck." 3 Mana
Voidterror: unchanged
Hellfire: unchanged
Shadowflame: unchanged
Pit Lord: 5 Attack, 6 Life, "Battlecry: Deal 3 damage to your hero, Shuffle a random card back into your deck." 4 Mana
Summoning Portal: unchanged
Bane of Doom: unchanged
Doomguard: 5 Attack, 6 Life, "Charge, Shuffle 2 random cards back into your deck." 5 mana
Siphon Soul: unchanged
Dread Infernal: unchanged
Twisting Nether: unchanged
Lord Jaraxxus: unchanged

Notes: Sacrificial Pact is changed so that you may trade your demon and get the heal, so as to not lose tempo by healing. Possible other fixes may include changing Bane of Doom to draw 2-3 cards if it kills a minion (it would slow tempo though quite a bit).