1. Get Mana Feed in a Shadow Bolt-spamming Affliction spec and show me how.
2. The Mana Feed changes were not buffs. They were to bring the Felhunter and Felguard up to the Imp and Succubus' level of mana returns, not push them over the other two. The reason is that Imp and Succubus can proc Mana Feed every global cooldown while the Felhunter and Felguard can only proc it every six seconds.
3. Even if they made Felhunter give you 20% mana back, it'd have a hard time keeping up with the Seed of Corruption spam.