There are two issues working against you here.
First, no matter what you've read, affliction isn't a good low level grinding spec. There's barely any meat in the 21 points it takes to get Siphon Life, the first serious mana efficency perk. 31 points finally gets you Dark Pact, but its pretty bleak getting there, frankly. Your second issue is Shadowbolt spam, which will rip aggro away from your pet while burning mana. Between these two issues, your downtime must be insane in both hp and mp.
I will always stand by Demonology as the low level grinding spec. It's assembly line murder. In the 21 points it took to get to Siphon life, Demonology gives you a number of survivability/dps perks to both you and your pet. A 3/3 voidwalker can hold chain more aggro than it can possibly survive while you roll dots on everything it touches. 31 points gives you Demonic Knowledge. It's not efficiency per say, but it's a decent increase in killing power. Basically it's Pet attack, torment, dots, next target, torment, dots, next target, torment, dots... for as long as mana holds out. You shouldn't be rolling shadowbolts unless you need dead asap. In fact, invest in a good wand. It's basically another dot without the mana cost. Sure you'll have downtime, but the sheer amount of mobs you can successfully engage simutaneously offsets that.
Now after 31 points if you want to switch to affliction, by all means do so. It's finally competetive at that point. But Demonology holds its own, even at higher levels. If you asked this question pre-tBC back in the days of 30 second soul link I'd probably say f*ck no to Demo. It was almost as bad as priest leveling ;)