I haven't leveled my Sorc as heals, but I HAVE leveled a Scoundrel as healer, and this has been pretty much my experience as well - once I unlocked and started using a DPS companion, my net survivability stayed about the same, but my kill speed went up immensely. And as for threat, I found that all the mobs that weren't being actively focused on by my tank companion would make a beeline for me as soon as I started throwing out heals and the AoE taunt wore off anyway.
Scoundrel, or in my case operative, leveling as healer goes faster since the ressource positive above 60 energy allow better dps from you imo.
I simply use shiv/backstab, and if Kalyo's health start dropping take a step back and heal more actively.
For sorc at that level, which ressource pool is based on how deep it is and not how fast it regen, you have to consider a force investment in dps is force you ain't getting for healing, possibly reducing the overall dps you can throw. Mostly healers will be using affliction very carefully, crushing darkness and force lightning when its an harder fight such as those you encounter in H2s (read longer fight). Ironically, such fight is where healer addind his dps would be nice, but also where its less practical to do so since it can compromise healing later in the fight.
It is true that healers sometimes solo H4, I managed hunger of the vr(something), last H4 of Nar Shadaa solo, howeer it was pre-1,2 when DI cast was actually 1,5 sec with force bending (no double dip tough, never approved using game bugs as a rotation) but if you have your legacy high enough and the cred for it, field respec is a priceless tool to subscriber playing an heal-able class, allowing them to freely go from dps for thrash cleaning to full heals for harder fight.
For those who are f2p or simply don't have access to that legacy perk, well its nit exactly that longer, but you will be far from the AoE burst dps does (on a thrash twoshot, where 3 level 50 normal mob died on a CL/DF crit, I got a reported 35k dps on MOX, since it was 13k damage aoe wise over 3 targets within 0,3 seconds from CL first hit)
For the record, dps can more than solo H2s as well, since they'll most likely have burned at least one strong before the shield fails. And you still have DH and DI to help out your companion. I normally keep recklessness availaible for such situations.