So I have tested Infernal in SoD and compared it to Felguard (Succubus can do pretty much same damage too) without any demonology talents and with decent gear (270 fire SP including demonic knowledge buff). I tested them both vs lvl 48 (I am lvl 50) wolf mob in Hinterlands.
Felguard did 48 dps (including his cleave - an auto attack that does +20 dmg every 6 seconds), Infernal did 60 dps including his immolation aura (25 fire dmg every 2 seconds). The infernal has 60 minutes cooldown, requires a reagent and lasts for 5 minutes and then requires to be subjugated again and can resist or break free sooner etc. He is not affacted by unholy power talent (+20% damage to auto attacks), he does not trigger master demonologist talent of any sort, he does not work with demonic knowledge and I think Soul Link does not work with it either. To my knowledge he does not scale with gear either (SP, stamina, intellect).
I suggest that:
1 - Infernal stats should be affected by Warlocks stats the same way other pets are when it is summoned (this should be probably snapshotted due to how the spell works).
2 - Infernal should be affected by demonology talents such as unholy power, fel stamina and fel intellect, probably snapshotted on summon. Maybe should even scale more than other pets to make him especially viable for demonology.
3 - Infernal should proc master demonologist buff the same way Felguard does, is should also proc demonic knowledge and work with demonic grace and all the other pet runes.
4 - Improved Subjugate Demon talent should be buffed when aplied to Infernal and Doomguard summoned by warlock. Right now 5 points in this deep demo talent reduce debuff to attack speed , casting speed and resist chance of the spell by 10% (from -30% AS / -40% CS / ? resist). Honestly I think 5 points should completely remove the debuff to be even worth considering. On top of it the duration of subjugation when applied to Infernal and Doomguard should be increased by 5 minutes with 5 points in, cooldown of their respective summoning spells should be significantly reduced with each talent point (maybe 5-10 minutes per point) and Inferno should became useable Indoors with 5 points in.
This change would make Demo lock a viable damage spec, would make Improved Subjugate Demon talent usefull and would give some love to this iconic Warcraft 3 ultimate.
Also this could be a rune or tome in P4.