Presence was the healing stat early in beta, they changed to the main stat for the class to cover both healing and damage, and for SI's, that is Willpower.
A Corruption Healer can do decent dps with Khem Val as companion. Use Khem to grab and hold agro, then dps while he tanks, and be sure to heal when he gets to a certain level. You essentially have your own mini-group. Be sure to keep him geared, and the best way is to grab the Khem Val specific drops from quests. Most of the time, they are good upgrades. Check every time though because sometimes those will be a minor upgrade and you might want to take an item for yourself.
I did notice in the last build that droid gear was rewarded in several quests, and some people did try using the ship droid as a healer. It was late in beta though, so that may be a good option if you want to dps or tank as an SI (tanking requires Assassin though).
Some of the heals appear crapptastic to start, but they scale well with level. The heals work together well. For a tougher mob, I would shield and toss a regen on (sorry, I don't remember the spell names), and then dps and watch, when Khem's health dropped slowly below 50% or if it started dropping fast when the shield fell, I would start the big heals. It's very similar to healing with a priest in WoW.
Oh, and for big groups of mobs, you may need to cc as well. The Sorc has good cc ability on top of everything else.
If you understand the fundamentals of group play (I know a lot new to MMORPG's won't, and don't worry, it comes with practice) and agro management, leveling a full healer SI with a geared Khem Val is very efficient and a lot of fun!!!! I won't drop a spoiler, but I will say that it was late in leveling before I changed my main companion.
Khem is a pain in that he hovers and stays close, and you always seem to be mis-clicking on him.. moreso than any other companion; but once you get used to him, he is one of the best solo'ing companions in the game, in my opinion.