Her mantra Q does ass loads of damage which is why I use it in conjunction with her W to snare and blast them in the face with it right before they are tethered. That's the point of her combo when building her for AP damage. I can do over 1k damage with only decent gear at mid level with her mantra Q alone and that doesn't include the added damage from the W. No comparison? I beg to differ given that I've taken out many a champion with that combo.
No no, I get that. I know it does more damage, though only by a couple hundred - substantial, but not enough to outweigh the substantial healing of the boosted W if you've taken damage at all (which, if you're in range to tether someone, you probably have). At 0 AP boosted Q does 225 more. At 500 AP that number only rises to 375. Boosting the Q is the maximum damage option if you can land both parts, but that pretty much requires having the target CC'd since it takes so long for the detonation to go off and if you're doing that CC yourself with the tether then you're probably taking hits yourself since it requires staying within a certain range of your target. Unless you're just killing off a runner, that usually means the life-stealing tether is the safer option. If it's a teamfight and your target is getting locked down by other players, while that might seem like a prime Q blast opportunity, a big team-wide shield and speed boost will probably have the greater impact on the fight overall.
I'm not saying the Q is useless by any means. I'm just saying that, most of the time, there's more utility and effectiveness to be had in boosting one of the other abilities instead of that one.