Is the shield/tank tree that bad for levelling?
The tank tree is perfectly effective at leveling, if a bit boring. Your first and second comp are a DPS and a healer, so you're set for your companions pretty much from the beginning. You'll kill stuff slower but still plenty fast enough, while having way lower downtime, not to mention shorter queues.
Assault is generally preferred for leveling because it's good for PvP and has excellent DPS, which means killing stuff as you level faster, even if you do have more uptime (though not that much if you run with your heal comp).