Since the mana pool of an imp is super hard to find details on, I've extrapolated a couple of numbers from level 63+ in tbc, as those were the only ones I could find.

An imp at level 35 should have roughly 1000 mana (I'm being generous here),

http://db.vanillagaming.org/?item=16317 costs 50 mana and does 33-38 damage, or 35,5 on average.

After you invest 3 talent points in to the imp talent, the imp does on average 46,15 damage per fireball.

Let's say the imp has 80 spirit, that means that the imp will do a total of 23 DPS until it is out of mana, at which point it'll do a whopping 6 DPS!

Now let's see how much those talent points actually gave you:

DPS before oom: 46,15 - 35,5 = 10,65 / 2 = 5,325

DPS after oom: 6,6 - 5,07 = 1,53

So realistically speaking, if you're being efficient, you just payed 3 talent points for 1,53 DPS. That's utterly useless and the imp is equally useless.

I strongly recommend you spec the spec suggested in my earlier reply, but if you want to play a demonology imp, sure, go ahead.

Oh and both the mana pool and the spirit of the imp is extremely generous and I've completely ignored resistance chance in these calculations, the actual DPS gain should be less than 1.