I leveled as full healer. It is tough until you get Ashara. She is quite honestly a better tank/threat holder then Khem. If you gear her up, she'll do enough melee dps to maintain threat, while you sit back and keep her upright and mix in some dot damage.
I find the tanks we get are terrible, not enough damage output to be worth keeping upright.
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.