I have heard a few mention that Succy is a dps increase, but ignore them. My Felguard is my demo minion, 200 dps diff is nothing and I have alot more usefull things on the Fel (stun, AoE).
Yes, you need to get improved soul fire. I am also specced into Corruption, and have Glyph of Corruption.
Make sure you are hit capped 17% for raids ( there was interesting post around before about hit cap not being king. Its worth a read
).
Macro in FelStorm and Rain of Fire, and make sure you use Demonic Empowerment when its off CD. This AoE is so OP, blizz have even addmited it needs a nerf, so use it while it lasts!
For single target, Macro in Demonic Empowerment to some of your rotation spells (Immolate, Shadowbolt, Soul Fire e.g.).
You may get errors from the macros so you will need to add in another code to make it shush. (Cannot remember it right now. At work atm).