Why are people stuck in their way. No, really?
Concealment is only great the first 30-36 levels or so, from there Lethality picks up.
By that time you'll also have Doctor Lokin, which furthers boost your performance. I would not recommend using Kaliyo at all. She never were any good tank to begin with, and only tolerable for healing operatives.
Also, leveling with a healing/non-tank companion rends Concealment semi-useless, since the tree beefs up Backstab and you won't be using it while tanking the mobs yourself.
To further boost your solo kill speed I'd recommend using Temple. Her dmg output can be overwhelming. However, for champs/HC2+ quests you'll HAVE to use Lokin, along with your amazing utility buttons (stun, shield, daze, etc).
While Conceal still has it's spot for PvP, I wouldn't recommend it for PvE, not even for the lower 40ish tier.
Lethality is also the only acceptable DPS spec us Ops have for endgame PEWPEW. Some may argue otherwise, that Concealment is indeed competitive endgame wise. Let's not kid ourselves: Operative/Scoundrel DPS/DD is rather low as it is, at least Lethality pumps out acceptable enough numbers to grant us a fighting chance... 'till that pesky Sniper/'Slinger or Merc/Commando shows up and steals our show.
Oh, and another thing: the tree is great fun, too! it makes us both a semi-ranger and a melee DDer/DPSer (unlike Conceal, Leth focus less on dagger attacks, and more focus on the blaster).
Regarding tanks: in general, tank comps are not recommended for solo leveling, unless you decided to level up as a Healer. Their dmg output is low, meaning longer time on the trash/NPCs, and they take quite a beating, meaning you'll have a lot of downtime healing yourself up after every fight. In my experience, and that of many others, a DPS/DD companion can "tank" stuff much better than the real "tanks", whilst they burn thru NPCs in seconds, too.
Healer comps are the greatest, tho: hardly any downtime, if any at all, good CC and decent damage. They prove the most effective.